Does the shape of the boat make it float?

The shape of the boat only affects how much energy is needed to move it (overcome friction) when floating on water. The boat has to have a high-enough profile so that when it is placed in water, it will not take on water. The amount of water displaced by the boat counteracts the weight of the boat exactly. When a 200-pound person steps into the boat, the boat will sink to displace an extra amount of water, the weight of which equal to 200 lbs. That is assuming the boat has not sunk yet.