well, havnt posted here in 4 months, so here goes...
the OFP game engine doesnt allow a boat to travel on land, to a certian extent - in the same way water planes dont work...
in ofp, its either a car, a boat, a man or a plane - so if u want it to float, it wont travel onland, of u want it to travel onland, it wont float....
i dont think there is ay workaround, although least a nice hovercraft to land marines from the wasp class carriers onto beaches...