Balsa wood. Just use reinforcemeant beams made of thickr or stronger woods. Find balsa wood in your local hobby shops.
You can make a boat with any wood, even ebony, which is denser than water. Ships are made of steel, which is considerably denser than water. Just try to make it leakproof, and large enough to displace enough water for its own weight, plus 4 pounds, plus some margin for freeboard. Balsa wood is cheap and easy to work with, but it’s not strong. A boat can’t drown, since it’s not air-breathing or even living.