This is an awesome adaptive robot hand that is open source and 3D printable - Yale OpenHand
It is based on the SDM hand, which is naturally adaptive by design and can easily and robustly grasp objects any size, shape and orientation from any angle; which would make it perfect for an ROV manipulator. Here's a nifty video of the SDM hand in action. btw, this hand has spawned designs that have won the DARPA ARM-H competition, and are currently being used by iRobot for their IED robots. So this hand rocks, and is quite field proven.
Since this hand is cable driven, all it would need is a modified waterproof 1-dof actuator, and PRESTO we have a spiffy ROV manipulator! I'm going to be working on making a nice liquid compensated manipulator based on this hand design, and Ill be posting updates here!
here are links to relevant papers: