All boats under a certain length are required to be virtually unsinkable, i think it's 20'? They have foam in them so if you get swamped you have something to hold onto while you wait for rescue. Over 20', well, it's going to the bottom.
There are two types of 2-part foam, the cheap expanding stuff and the really expensive stuff that can be in the water for years without absorbing water. People also put other stuff under there to provide some flotation.