Yes, but birds are very light in general. Most of their volume is feathers and they have a low bone density to boot. As the result, they have a very hard time diving, and have to either dive at high speed or paddle really hard to stay underwater.
And regarding boats, it depends. Do you mean completely empty passenger boats? Then yes, their density is very low by design, because they are mostly empty on the inside. When fully loaded, a commercial cargo vessel, is 80-90% under water.