I am no professional, but I'd say that when it comes to the term 'floating' it does not matter. However, flat would mean that there is less water displacement, therefore more molecules floating.
When it comes down towards the practicality of a boat's hull (body), concave (round) would be required for stability and the streamline aspects. It will make it easier to steer and you have more 'body' IN the water for added stability.
Some might say it will matter less on rafts, but I'd say the basics stays the same.
round
It is the boat's draft.
It is the boat's draft.
It is the boat's draft.
It is the boat's draft.
It is the boat's draft.
The space between the boat's keel and the bottom is the draught.
The distance between the bottom and the keel of a boat is the draught (British English).
Draft.
Draft.
Draft
Draft.