The layout of the boat, as I understand it. The euro style transom has hatches and rigging holes behind the full height transom. It near about has to have some to facilitate the rigging. I have a Whaler with a similar transom design. It has a large fiberglass hatch giving access to the batteries on the starboard side (with a nice rubber gasket) and then a rectangular hatch at the bottom of the splash well. (yep the bottom of the square splash well and it is kinda cheap looking)
I am confused as to why they were not anchored up while diving. A motor would need to be running with someone at the controls to keep the boat close AND when the problem was detected the motor would not be shut off. Why not put a couple guys on finding the leak, isolate the battery issue and get frantic about bailing. With both batteries submerged, because they are buried in the bilge and access to the bilge already underwater I am sure they were limited. I would have at least tried something. If the bilge pump was on, the leak was substantial and probably killed the battery but that would have been noticed long before the cameraman made it back onboard.
The Jackson's in the "Merlin" live around the corner from my parents in Suwannee. They are the neighbors that I referred to as catching nice Red Snapper this past weekend in the fishing section under "Scallops anyone?" Small world unless your floating along looking at the bottom of your 30 footer somewhere in the Gulf.
Glad they made it home safe, there was some editing involved so they probably tried more than the video showed. Easy to play armchair quarterback, now it is time to go and inspect them hatches, test the bilge alarm, both pumps and consider another battery that isn't below the deck.
Good battery!!...