In the past few years, we have covered plenty of awesome underwater robots here. The SherpaUW is also worth a look. It is a hybrid walking and driving rover with 4 x 5 DOF legs and a 6 DOF manipulation arm. The joints are all sealed for secure operation under water while the arm is pressure compensated.
The actuators are rated to a depth of 100m. This robot can perform extended missions on the sea floor.