The only way that I could see a platform floating is if it caught an air pocket underneath it due to the lip, which I doubt would happen or stay there. I would think that assuming there is some bouyancy to a platform it would be so minimal it hardly even worth counting. For example (not actual ratings) if the platform could hold 500#s out of the water, it could hold 510#s in the water. This is assuming your not moving. If your moving, the water support would be gone, back to dry weight. The small amount of difference is not worth counting and more appropriately should be called margin of safety.

When it comes it comes to the kids, I never have worried. As for adults, is there room for more than 2 anyway?