Hi @Medemt,
I believe this thread is where the main relevant discussion has happened on the forum, so I’d suggest reading through that if you haven’t already 
This is likely best addressed in the context of specific requirements. A single actuator manipulator like our Newton Gripper is relatively simple in terms of theoretical design, and quite simple to control because there’s only one thing that can move. As additional actuators and motion axes get added the complexity increases, both for the mechanical design (e.g. determining torques in different orientations) and for making a control system that allows the desired type of control (e.g. avoiding self-collisions, making control intuitive).
The general theory behind robotic manipulator design and control is likely best found in general robotics textbooks and internet searches. If you have a particular set of requirements then we may be able to help you determine particular mechanism, actuation, and/or control components that are relevant to achieving that.
It’s perhaps worth noting that while underwater operation adds some challenges in terms of pressure and electrical insulation requirements, the rest of the theory is general to any remotely (or autonomously) controlled robotic arms