Walkthrough

This is a walkthrough for the game 'Mermaid Swamp' I will write a warning right before a spolier as to not ruin the experience.

The spoiler warning will resemble this: WARNING: SPOILER AHEAD!!!!

Spoilers currently look like this, so avoid these boxes if you want non-spoiler.

Note: This walkthrough was not created by me, it came with the game with only some slight edits that will be changed later

Intro

Just play until you find the map proposal

Day 1

Check on Yuka and you'll hear a sound. Check the dresser to find a key (bedroom).

Talk to Yuuta, who's looking at the painting in the 1F dining room. Follow him to the 1F southeast hall, then again to the 1F southwest hall.

Read the book in the study for a hint. (Seitaro is also there during the day, so ask him about anything you come across.)

Hint: "The middle of the world" refers to the paintings in the symmetrical rooms to the upper-right of the map when you enter the second floor.
Answer: Check the shelf in the right room and move it to find documents in the wall. Seitaro can read them for you.

Use the bedroom key on the middle door at the south end of 2F. Check the bed for a wire and the right shelf for a note...

WARNING: SPOILER AHEAD!

Hint: The note gives you directions for a certain room,
and Rin is correct that the circles refer to eyeballs. 
Where have you seen eyeballs that big?

Answer: Go to the room with the two fish tanks. 
Start in front of the left one, and take 5 steps down,
2 steps right, et cetera (the arrows represent the 
direction you walk). You'll end up on a tile on the
right side. 
Check the floor there to find a newspaper article.

Use the wire to lift the latch on the Japanese room looked from the inside on 2F. Read the diary.

Check the pot in the upper Japanese room. Go to the 1F kitchen and check the cupboard for cooking chopsticks. Use them to pull a key (master's room) out of the pot.

Go down into the lower Japanese room and check the closet for an old piece of paper. Seitaro can read it for you. Just a warning: When you enter, a jumpscare pops up for a moment.

After doing everything you can, Seitaro will talk to you in the hallway. After getting the lantern and lighter, go to Yuuta's room. After the quick dialouge scene, go back to your room.

Night 1

Use the lighter to open the 2F door with wax in the keyhole. Get the letter from the desk and read it.

Go to the piano room and check the left child's painting for a key (storeroom). Use it. The door is near the entrance on 1F. Inside the room will be firewood on the right side. If you get some firewood, the game will force you to light the fireplace in Yuka's room, therefore killing her and Yuuta. If you're going for the good ending, don't pick up the firewood.

If you light the fireplace in Yuka's room: Yuka will die. If you don't light the fireplace in Yuka's room: Yuka and Yuuta will live. Note that you can basically delay lighting it for a fair while (that is, until just before the point where it starts making a difference).

Leave the main building and go to the old mansion. The door has a pass code.

Hint: The diary in the master's room says that he locked it "with that sorrowful year and month as the key." Answer: 197208, from the date on the article, 1972 being the year and 08 representing the month.

Go up the middle stairs, and when you hear a noise, check out the window. Return to the swamp, then go to bed after the cut scene.

Day 2

Check on Seitaro. Rin will wake him up.

Go to the abandoned building and get the key (study) from the bath.

Unlock the study (west side of 1F). Push the table from outside up to the shelves to get the spring.

Return to the main building's storehouse for a mop. Use it to clean the sludge on the stairs. Go into the makeup room (west side of 2F) and read the writing on the mirror.  

Find the clock on 1F and use the spring to start it. Set it to 8, as indicated by the mirror, for a bottle of liquid. You need to use the up and down arrows on your keypad for this  

Read the diary in the nursery (west side of 1F). Go to the room with eight fish tanks (east side of 2F) and reach into the second from the bottom-right for a button.

Ask Seitaro in the study about the bottle. Use the bottle to melt the nails on the boarded-up door and get the planks. Use them to enter the door with the large hole in front (west side of 2F).

Get the golf club from the bag and read the books.

After the call, go back and check on Yuka.

If you lit the fireplace, Yuka dies. Yuuta will run off, eventually stabbing himself in the kitchen.
If you didn't light the fireplace, Yuuta will chase you and you have to destroy all the Ophelia paintings: first the six in the main building, then the one in the old mansion.

Go outside and head left into the mountains. Keep walking. As Seitaro, do the same.
In the village as Seitaro, knock on the doors, then go left to the gravesite. Read the sign, and go left again to find Rin.

Jump in the swamp.

Night 2

Enter the bedroom in the hallway between the west stairs and the entrance hall. Read the diary on the book case. (You might not need to; if you already read it before, then you should be fine.) It will be the first door you pass from the entrance.

If you lit the fireplace: return to the kitchen and collect the knife beside Yuuta's dead body. If you didn't light the fire: return to Yuuta's room and collect the knife on the table.

Go to the bedroom in the old mansion (east side of 2F) and cut open the left bed's pillow for a key.

Check the fireplace up the middle stairs to find a passage down. To enter, you need these three objects:
rope: In the clothing room (2F east hall), entered using the key from the pillow.
picket: The fallen stone picket outside the old mansion. The first on the right as you approach the old mansion.
hammer: In the master bedroom in the new mansion (bottom-left on 2F).

Take the piece of paper from the table and leave.

Day 3

Go down the stairs.

WARNING: SPOILER AHEAD!!!!

Accept: Game over.
Refuse: Start running.

Go into the leftmost room on the second floor.

WARNING: SPOILER AHEAD!!!!

Dodge: Seitaro dies.
Don't dodge: Seitaro lives.
Run out of time: Game over.

Depending on your choice, you either talk to Seitaro in the old mansion's study or check the gleaming book in the old mansion's study to learn how to tell directions.

WARNING: SPOILER AHEAD!!!!

If Seitaro is dead: Click on the gleaming object 
beside the memorial in front of the main mansion
before returning to the old mansion.

Check the map in front of the swamp to find your way.

Open the closet in the old mansion's servants' room (east side of 1F, with four beds) for the boiler room key.

Go into the boiler room outside the old mansion and get the engine key. The boiler room is the shed beside the old mansion.

Use the engine key and button on the motorboat outside the old mansion to cross the swamp.

WARNING: SPOILER AHEAD!!!!

Hint: Check the map for a clue for what to do on the other side.
Answer: Check the ground to the right of the largest brown rock.

Endings

Yaobikuni (Lit fireplace, dodged ax)

If you lit the fireplace in Yuka's room, she will die. Yuuta kills himself out of despair, though he was possessed by one of the Tsuchida men's ghosts at the time. Seitaro covers his body with a cloth and tells Rin not to go inside the kitchen. If you dodged Seitaro, he will fall through the floor, resulting in the axe falling in his stomach. He jumps in the swamp afterwards. Rin goes to the secret trapdoor underneath the earth, and finds out that she is locked in by the Old Man. She becomes hungry, and grabbing a pickaxe, she breaks one of the mermaids' aquariums and eats its corpse. Several days pass and the Old Man unlocks the secret room, to find Rin is still alive. He proceeds to tell her the legend of the mermaid, eating its flesh for immortality. He lets her go, because even if she tells the story, no one will believe her because she is utterly insane. Rin is last seen at the forest, laughing maniacally as she runs deeper into the woods, convinced that she will never die. Assumed that she will die.

Forever Deep (Lit fireplace, did not dodge ax)

because Rin cut him off, telling him to make up his mind. Seitaro finally gives in and tells her they were indeed monsters; but after that, they found out they couldn't leave the swamp. Afterwards, it is assumed that they both committed suicide by drowning themselves in the swamp.

Secrets (Did not light fireplace, dodged ax)

If you did not light Yuka's fireplace, she and Yuuta will live. In this ending, if you dodged Seitaro, he will die, leaving only Yuuta and Rin to solve the problem. However, Rin hears Seitaro calling her over from the other side of the swamp, though Yuuta hears nothing and thinks Rin is hallucinating. Rin tells Yuuta that she is going to be with Seitaro, and jumps in the swamp. Yuuta jumps after her, but fails to save her. He is the only one who can save Yuka and himself (and the mermaids)...but is promptly shot, presumably by the Old Man.

Underwater Dream (Did not light fireplace, did not dodge ax)

Yuka and Yuuta lives if you did not light the fireplace in Yuka's room. Seitaro lives if you did not dodge him. Rin kicks him repeatedly in his stomach until he passes out. When he wakes up, he is returned to his usual self. Rin and Seitaro find the underground room. Rin panics, and asks Seitaro if they are monsters. Seitaro replies no; instead they were corpses of women. Seitaro finally solves the mystery of the mermaid legend.

A man was walking by the beach, when he saw a beautiful woman, maybe a diver, swimming along. The man fell in love with her and abducted her to his home. He loved seeing her in water, thus he put her in a tank. The girl suffered health complications after being in the water so long that she finally died. The man couldn't stand to see her rot, so he went to a last resort and preserved their bodies. He tells Rin about the documents with the preservatives she found in the mansion, and that the preservatives were used in the tanks with the large tubes on the wall connecting it. Rin asks why there were so many corpses, and Seitaro replies that the man couldn't have just one girl. He abducted more girls from a nearby village, did the same to them as he did with the first.

Seitaro leaves the room, and when Rin was to, the mermaids tell Rin to take them back to the earth. Rin spots blasting dynamite outside the door, and tells Seitaro to help her with the mermaids. Seitaro does. Afterward, Seitaro asks Rin if she is hurt, but Rin is surprised to see tears falling from her eyes, and says that they were not hers.

They go back to the Old Man and Rin threatens to call the police, but Seitaro stops her. The Old Man confides to them everything that had happened. He says that for the short remainder of his life, he would pray for the deluded men, the mermaids, and most of all, his family.

Rin and Sietaro goes back and Yuuta tells them that the car is working again. The two boys carry Yuka to car and goes back home. While riding, the blanket falls off Yuka, revealing that she is normal again, and Rin is overjoyed.

(Help wanted over here on the endings!)