Man, Terraria is so cool. It's hard to get all my friends on to do a playthrough, and they usually burn out before we even finish!
I've had an idea that a really cool playthrough would be manually flood every single tile with a tool like tedit, haven't figured out how to do that though.
It would totally change the balance and play of certain items. You'd have to make potions before you could even begin to venture underground.