A heavy rock sinks, but a boat, equally heavy, doesn't! Neat! So what gives? The Biggest factor is the contact area between the boat and the water (the area where the boat and water touch). The secret to floating boats is a problem of spreading the weight over a large area of water. If you think of a bunch of water molecules trying to hold up a boat, they're pushing against the boat to keep it on top of the water. If the contact area between the water and the boat is larger (a large area of the boat is touching the water), more water molecules can push against the boat, so they can keep it afloat. You can think of it like lifting furniture. You might not be able to lift it alone, but if several people help out, each person lifts a part of it. Boats tend to have big flat surfaces on the bottom (the hull of the boat), which helps them float. Although they're heavy, they spread their weight across a large surface in the water.

There's a second, and more complicated, part of floating that has to do with pressure. If you've ever tried to swim to the bottom of the deep end of a pool, you know that your ears get sore unless you equalize the pressure in them. This is because the pressure of a fluid (like water) increases as you go further down. If there's a big object in water, the bottom of the object (in deeper water) tends to get pushed up by the higher pressure, and this also helps it float.