The buoyancy physics is not yet implemented in UE4, but it should be straightforward to get the height data and calculate the intersection of the height field with, for example, a convex hull. When we get some extra time I hope to publish a boat sample using a rigid body for the boat hull, and a pawn controller that applies buoyancy, drag, propulsion, etc., similar to how the vehicle sample uses a pawn to create a PxVehicle.