I'm a physician (inactive license as Surgeon in the USMM), not an electrical engineer, so these thoughts are tentative, not authoritative. But I think the ocean exchanges negative ions with the rest of the environment similar to dry land.
Lightening has been shown to be the result of a buildup of (-) charge in the earth, relative to the clouds full of water droplets which have a (+) charge due to electrons being stripped off during their violent uplift. This eventually leads to enough potential to cause a breakdown in the insulating properties of the atmosphere and current flows suddenly and violently.....in an earth to cloud direction contrary to popular myth.
As any mariner knows, lightening certainly can be seen at sea, both between the water and clouds, and sometimes through vessels afloat on it. In fact, static discharges of less than explosive degree (St. Elmo's fire) are associated with ships in most people's minds, though they occur on land as well.
I have no idea how the (-) ion potential of the sea compares to the land, but certainly the flux is there. So, if you're on a metal ship, just make sure your bunk is grounded! If you're hull is made of something relatively non-conductive, ground your mattress pad to your through hull bonding system that protects your metal parts in contact with the sea from electrolysis. That should give you all the (-) ion flux you need. Now, will it really do you some good? In my medical opinion, the jury's still out on that one. Studies reported in that article are encouraging but not enough to persuade me yet.