Good Game
Like many said, it is a bit like Motherload, but it still has an original concept. If you do make a second, it would be a good idea to include a storyline or something like that. Also, instead of diving down to the lowest caves only to find another $2000 or so I won't be needing anyway, why not make it so that the lowest levels house special upgrade parts that can't be bought, like salvaged hull plating from the Titanic, or a reactor from a nuclear sub? This would make the whole risk of diving so deep down more rewarding.
To all of you people looking for advice on what to buy first: plating and oxygen are important, as is a spotlight. Battery... eh, not so much. If worse comes to worse, you can always pedal. ([S] and [D] repeatedly.)
Oh, and by the way, to all of you whining why the sharks would attack a submarine, though I agree that this is not so realistic, maybe the sharks just think it's a giant Twinkie =)