Buoyancy is the weight of the water displaced by an object. An object is buoyant if the weight of the displace water is bigger than the weight of the object than an object will float. I added hollow pontoons so that the boat is more buoyant because the air weighs nothing. The pontoons also provide stability so that the boat doesn't capsize as easily.
the insides of my pontoons
My boat is good in holding coins, it carries about 5 or 6 coins, witch is well above the expectations. If you make small waves my boat will float just fine, however with bigger waves a bit water spilled in the boat but stays afloat. Something to fix that would be to put the cargo hold to the boat higher. I also could make the walls thinner so that the boat is more buoyant.