Will this be in conduit?
One option is submersible pump cable, but its not jacketed. I would also suggest XHHW-2, its a very tough insulation and much better than any THHN/W product which is PVC insulation. If no conduit then it would have to be direct burial rated.
[No it will not be running in PVC it will be running on the floor of the sea. There must have some special cables for this purpose ?? ]
I have tried to send an email to Carol at 'email@example.com' and no reply ??? Can you assistI worked on a marina that had a previously installed set of type W cables that were installed on the ground going into the water and were submerged for about 15 - 20 feet all the time. I know it was inspected and passed.
Maybe Carol will help you out (they say to call them) . https://cdn.generalcable.com/assets/documents/North America Documents/Information-Center/White-Papers/Cord/MarinaDockPowerWireWizardSellSheet4_09.pdf