Phil. Making a small sea nimf may be a bit tricky, getting it to float with some degree of stability in rough waters.
I might suggest something with a flat bottom.. how about a civilian ferry, or a tank landing craft..so that his favourite cars/tanks can be put inside? or a cross channel type ferry, or a catamaran of some sort. These are fairly simple shapes.
Something along the lines of my "Devastation" could have the hull cut from a single piece of styrofoam, for instance, and if propulsion were needed, you could fit a submarine power unit underneath to power any of them..a little self contained "torpedo" that fastens on the underside..acts as a keel as well !!
Have a look at, for instance http://www.hobby.uk.com. They do some good stuff, and a range of "junior kits" that might suit the bill.
Everyone who makes model boats should have a link to hobby`s on their favourites! dead useful.
Ashley