Popcycle sticks; glue them together using a water resistance glue. You want to leave just a little crack near the water line that will permit water to seep in. Inside the boat; about 3/4 of the way up from the bottom of the boat you want to build a self (kinda like a cage) that you can open. Depending on the size of the boat (your going to have to test this) place Alka Selzter tablets. Now when the Alka Selzter tablets get wet they will start providing air bubbles which when trapped in a well sealed upper part of the boat will make the boat to rise. Not completely but to the point of the water line where the water seeped in. Therefore; a floating sunken boat...
Well, I don't have any specific ideas but it sounds like you want something that will attach to the boat and let it float for a bit. Then this object reacts with the water to pull it down. At some point, it must detach from the boat, and the boat then floats back to the surface...??
So maybe a boat made of cork, and...I have no idea.
sprite, or any clear, carbonated liquid, and raisins. the carbonation in the liquid will pull the raisins up, then the raisins weight will pull it down.