I could just build an "observation class" ROV ... but where's the fun in that. I want to build an ROV which can prod its environment. To do that I need a robot arm. The easiest way to do that was to buy a basic robot arm kit on eBay, strip it down, and rebuild it with waterproof servos. Now, these servos are not rated to go very deep and this arm may fail spectacularly .... but it's a place to start .... so ....

I bought the kit here, and followed the instructions here. I stripped out all the steel parts and replaced them with stainless steel, stripped down the grabber and rebuilt that with nylon washers (which greatly improved the smoothness). The assembled kit, mounted to the frame, is shown below: