I’m thinking about making one so transportation will be easier. Any advice?
I’ve used these http://wet-connect.com/pages/il/il_all.html wet mate connector, used by diver on their communications system, I’ve also threaded the tether through hollow polypropylene rope and spiced a snap shackle on for strain relief, that way there’s no tension on the tether and I can use the rope to haul the ROV back to surface if there’s a problem without the risk of the connector separating.
Those connectors are good for professionals, especially when going thousands of feet deep, but kind of expensive for hobbyists. These connectors are cheap and good enough for shallow work: https://www.amazon.com/VIMVIP-Female-Waterproof-Connector-10Pieces/dp/B015C9Y11C/ref=sr_1_34?ie=UTF8&qid=1490639035&sr=8-34&keywords=led+connector+2+pin
If you want something in between these choices, check out the Souriau hi-seal line of connectors.
I’ve been using them for years in hard Marine conditions but not actually underwater. IP68 rated, even when un mated.
Here at OpenROV we use connectors similar to the ones that @Douglas_Butler referenced above. The ones we use are made by LEDJump and are also available on Amazon.
They are not totally waterproof, but seem to hold up well, especially if you pack them with dielectric grease before putting them in the water.