After a month away, finally got the ROV back in the tub today.
First, the good news. The new tubes sizes and arrangements means the ROV is much less buoyant than before. I was able to remove all the extra lead weight which still left the ROV slightly negatively buoyant, so I'll need to look at a way to neutralize that (maybe some sort of foam). However, despite that, I was able to make a start on re-calibrating the PIDs for the thrusters.
But .. a little way into this I noticed that the thrusters on one side of the ROV were no longer running. Quickly hauling it out of the tub I was horrified to discover water was in the battery/ESC tube causing a short, shutting down the controller on one side of the ROV. I guess the new fuses did their job.
So now, I'm just drying things out and waiting to see what damage is permanent.
Behold - soggy ESCs :-(