We took our OpenROV for its maiden voyage out on our local pond yesterday ...
We had threaded the tether through the inside of hollow core 1/4 polypropylene rope, which takes some time but it's nice to have something stronger to pull the OpenROV back with instead of its tether.
In the bathtub tests it sort of seemed like it might be floating (wasn't clear), but it the pond, it quite clearly did NOT float.
We'll investigate options, e.g. maybe taping some foam every 10 feet or so, but I just wanted to get this info out there so others could maybe try larger thickness rope.