8 feet (approx. 2.5 m) sit-in kayak...you sit in it, and your upper body and head are (roughly) where a battleship might have its superstructure. Use lightweight materials to make the 'turrets', etc. Get an old trolling motor, controller and battery, and you have the 'makings' of a 'battleship' that can patrol your local lake for hours on end.
If your head sticks up a bit too much, get an old full-face motorcycle helmet, and add a few bits to it to disguise your helmeted head as a gun director/fire control radar!
Tom