I've used them before on surface boats and had the main battery drop down enough to cut the power to the radio...result, a stranded boat. In a sub that could mean a sub down on the bottom with not enough power to get up again, I guess.
My new submersible will be a dynamic dive boat to start with, so the problem isn't quite that bad, but I do plan to fit a pump and bag later, that's when the problems could arise.
So, what do you think...fit an extra battery pack for the rx? Or go with the BEC?