i would completely agree with Jon.
most robot builders fail on their first project because they make things too complicated too soon.
my first robot was doomed to failure because i made things too complicated.
i decided i wanted the bot to be able to map the room and plot a path so i had loads of sensors and a lot of mathematical routines running on the processor.
the main problem though was i just picked motors that were too big.
the battery life sucked, it went too fast to be controlable and the interference caused by such big motors made my controller behave eratically.
my 2nd robot was far more successful after applying the leasons i learned the first time.
it was driven by 2 smallish servos and had only limited sensors but could still find the beacon on it's battery charger within a few minutes from anywhere in my (small) house.
you'll hear loads of similar stories from amateur robot builders.
start simple, learn and progress.
now with more experience, i'm back trying to make a fast robot that can map the room but i could never have done as well if i had not built more simple bots in the past.
i'm doing much better this time round so don't give up on that underwater bot entirely....
check these for future inspiration:http://web.ticino.com/Submarine/http://web.ticino.com/Submarine/E1_page1.htm