I've seen mods for the roombas before, where the robots are water sealed, and then the wheels are turned into propellers. My idea is that I could water proof the robot in plexi glass, or something, and then a net on it, to collect all the leaves, dead bugs, and other stuff that collects on the surface of a pool. normal pool cleaners only vacuum the floor of a pool, and it takes days for floating leaves and other grime to sink to the bottom. With a robot like this, I could just set it in the pool, and let it clean everything up. What's nice is, when company is coming over, it's embarrassing to have a dirty pool...this modification would allow you to quickly clean up your pool-even on short notice.
Update (for some clarification): What I would want to do, is make a case for it, so that you can simply remove the wheels, and connect the propellers/steering device to where the wheels were, then just place the roomba in the watertight case, and it would be able to swim around and clean!
Also, i'm working on some sketches of how it would work. I'll post those when they are done.