Well, the moral obligations are simple: if you can bring someone back you probably should. And if you can't the fall back position is that you carefully mark where you left the body behind so it can be retrieved at a later date and that you make it your priority to notify the next-of-kin and the authorities truthfully about what happened.

And if you're within radioing/cell phone distance of help that you ask for that help.

If you're 3 hours away from the shore and you're most likely purposefully that far out just because you want to dump the body of someone you killed in deep water then of course none of the above applies to you.