The robot would be an underwater vehicle with two emergency "parachutes" to come back up when needed (when battery is done, or if no signal is received withing a certain amount of time). It should be lightweight so low cost of movement, and hydrodynamic, but heavier altogether heavier than the water,
its size, so that it stays on the sea bottom. It would need the following systems: (Perhaps, if it's lighter than water, a bag of sand would be used and released (or added from the sea floor) as needed.
an anchoring system - in case it needs to stay put.
camera(s) and image transfer system (via sonar)
emergency buoy on thin fishing thread - first emergency stage, if something goes wrong.
robot hand(s) that can grip, turn, push etc. And robot tools to be used by the robot hands.
powerful LED lights.
A long electric wire version would also be available for smaller underwater distances.