That must be a very small boat to only handle that much weight. That will BARELY handle a single AAA battery before swamping. You may want to look at a larger model to make into an R/C. I've seen the 1:350 sized Tamiya/Academy models converted to R/Cs and know, first hand, that the Tamiya Bismarck comes with a motor and gearing and prop shafts to make the boat runnable, just not R/C. With some minor modifications, it can be made fully R/C controllable as can be seen here:
http://www.rcgroups.com/forums/showthread.php?t=925895