Just create a Shrinkwrap of the Assembly and merge the entire assembly into a single solid part, if its comepletly submerged, center of mass of that single solid part (shrinkwrap) is the center of boyancy. if only part of it is submerged, make a datum plane at water level,solidify(cut) the portion above that plane and then calculate the center of mass which will be the center of buoyancy.
Edit: if you have enclosures inside your assembly then you have to fill them before you can calculate center of buoyancy.you get the correct results only if you solidify the volume of displaced water.
Edited by: solidworm