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 proporsal

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 on 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 right, etcetera; 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. (when you enter, a jumpscare pops up for a moment, justa  warning)

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 you're room.

Night 1

Use the lighte r 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 painting for a key (storeroom). Use it (the door is near the entrance on 1F) and get some firewood.

If you light the fireplace in Yuka's room: Yuka and Yuuta 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 passcode. 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.

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 scene.

Day 2

Check on Yuka. Wake up Seitaro.

Go to the old mansion 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.  

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 hole in front (west side of 2F).
Get the golf club from the bag and read the diary.

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 left into the mountains. Keep walking. As Seitaro, do the same.
In the village, 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

Get the knife. (Depending on the path, it's either in Yuuta's room or the kitchen.)

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 in the large hall (middle stairs) to find a passage down. To enter, you need three items:
Rope: In the dressing room (2F east hall), entered using the key from the pillow.
Picket: The fallen stone picket outside the old mansion.
Hammer: Toolbox in the master's room in the main building (bottom-left on 2F).

Get the map in the secret room below the fireplace.

Day 3

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.

Talk to Seitaro in the study, or check the gleaming book in the study to learn how to tell directions. Check the map in front of the swamp to find your way.

Open the closet in the old mansion maids' 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.

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 big brown rock.

Ending's

Yuka, Yuuta, and Seitaro die: Yaobikuni
Only Yuka and Yuuta die: Forever Deep
Only Seitaro dies: Secrets
All survive: Underwater Dream