I do not know of anyone who is yet to 100% successfully create or modify a standard RC servo such that it can handle any sort of practical depth long term. I think most peoples attempts involve filling the servo with oil.
I have seen some good gripper designs use peristaltic pumps. Have a look at the more recent posts in this topic OpenROV Gripper, Grabber, Manipulator
In the last few weeks I have been working on a linear actuator/gripper design using magnets. See the video i just posted showing the first open and close test.
I’ve used some neodymium magnets and have been able get a linear force of 1.8 kgf before the magnetic bond breaks. Inside I have used a gear motor from ebay like the one in the photo below. All of the yellow is 3D printed ABS.