Being a 1/720 scale you may not have to cut the hull. If the base is deep enough you may be able to build the water up to the warterline of the ship. This would also give you an option if you wanted it to be in rough seas. To either raise the bow or stern w/o have to worry about the cut.
If not deep enough you could use a scroll saw or one of those Jewelers saws. The small thin blade is long enough through both sides at once and easy to control.
Hope this helps
Paul