Sorry if I'm saying a stupidity, but my English is not so good to follow so complex conversation, but I want to say two things:
Of course there is air in sea water. If not, the fishes couldn't breathe.
Another thing: the phisycs on the air could not be applied to water. Both are fluids, of course, but liquids (water in this case) are much more dense and complex than the air.
In fact, there no exits a unique formula for the speed of sound in the water. There are one for each place, even for each season, and all this formulas are made by empiric experients, no theorical.