mmm i couldn't really tell you as i have never had a vehicle thief in any map i have played on yr or ra2 the only game i remember a thief in is ra1 terrain sun or something like that but he could only steal money from the ore refinerys i dont remember him being able to steal vehicles though.I would think not as if my memory sieve's me right the thief can't swim unless you mean when the neval unit is near the shore line and if so i dont understand why you havent tryed to steal a neval unit while you were playing the game as im sure you would of got your answer there and then.I could have this all wrong and what your really asking is it posible to make i thief steal a neval unit.if that is what you really was asking me then again i havent a clue but if you want me to be honest im sure it would be possable to do, as if the thief can steal a land vehicle and as a neval unit is classed as a vehicle so i do see why not, but how to go about making a thief steal navel unit i honestly havent a clue.
sorry i couldn't help :/