Tricks&techniques to be used to put manual bots under water!!

hello...

I am little bit curious to know about the mechanism in which a manual bot can collect balls placed at different positions inside a water tank....

please can anyone of you help me out to solve the problem??

so my question goes like this..

How to make a bot waterproof(avoiding any shortcircuit)and the mechanism to be followed for the collection of balls present at different depths inside a water tank?And which one is a better idea:
making a bot work on 220volt directly or through battery?

eagerly waiting for your posts!!

proud to be an CEan