Short version : stop the madness. Keep your lens out of salt water. Period.
Salt water is corrosive and fairly close to the worst thing you can get on a lens.
Submerging any lens is an appallingly bad idea, as even sealed lenses are not designed for that. They're designed for, at worst, splashes and typically for light to moderate rain. You'll note that except for specialty products (specifically designed for underwater) manufacturers don't generally specify in any detail how reliable seals are on lenses or bodies.
If you need to submerge a lens and/or camera into salt water "frequently" then I'd strongly suggest investing in a proper underwater case.
So if you submerge a lens in salt water the very least of your worries should be the effect on the coatings.
And the same goes for cameras, BTW. In fact it's sometimes worse as salt water and electronics (which are also in most lenses) do not mix well.
For what it's worth I don't think the coatings are soluble, but, again, salt water is corrosive and, when it dries there will be salt on the lens, waiting to be rubbed by someone and this will erode the coatings. I'd recommend using a lens pen to clean it.