Simply build a robot that works on land and put it in a big waterproof box with some weights and dump it in a lake. Remember that the total weight of the box with contents needs to be greater than the weight of the water it displaces. The robot will then be an under water robot without damage to the robot. If this is not what you need, perhaps you are missing a few details in your problem definition. Part of engineering is a clear and concise definition of the problem. Am I missing something here?
As you probably noticed from other posts, you need to define your specific problem and your specific question--not simply ask someone else to do your work for you.