thanks for help i have seen it earlier also.but i want underwater switches not waterproof switches.they will not work in undewater
hmm, unless im mistaken, anything that is "waterproof" can be used "underwater". Of course, with your robot, the electronics must be contained in a water-tight container...the switch I linked to in my last post would work, if you took the leads of the switch and ran them through to the water-tight container and sealed the holes with some underwater rated silicone sealant.
Or maybe you have an underwater remote control type switch idea...
perhaps you could use a reed switch. have the reed switch located inside the robot attached to one of the walls (also with a piece iron to hold the magnet on). Then use any magnet to extenally turn on and off the robot.