Has a similar problem quite a while back. While heading in for a harbour raid, but still way out in deep water, I'd suddenly plummet to the bottom and implode for no reason.....
reload back at Pearl and try again.....
get to a similar point, and sufer the same again...
reload back to Pearl and try again.....
Finally after many reloads, it occured to me that I should try saving a little closer to my destination to at least shorten my trip each time, and for some unknown reason it (the crash dive of doom) never happened again. Never worked out why, or what caused it.:hmm: