If the downward gravitational force is less than the upward buoyancy force then the object floats, otherwise it sinks. That is, if an object weighs less than the amount of water it displaces then it floats otherwise it sinks. A boat floats because it displaces water that weighs more than its own weight.

Why do boats float?

The air that is inside a ship is much less dense than water. That's what keeps it floating! ... As a ship is set in water, it pushes down and displaces an amount of water equal to its weight.

How do boats stay buoyant in water?

They float thanks to the displacement of water and the resulting upward force created by that displacement. Since a boat's density is much lighter than the ocean's density, a little upward force is all it takes to stay floating. Even for the heaviest of ships! And that's how you float a boat!