No air tanks? What, are you snorkeling?
Underwater sex would drown you before you ever hit climax, especially if you neglect to use air tanks! Even with them it would likely be fairly uncomfortable having to breath through the mouth piece, and, inevitably, you would inhale some water which would feel unpleasant. Also, you have to account for the water being cold which is likely going to reduce your sexual drive. And we haven't even covered water pressure or currents or the exhaustion that comes from swimming!
So what do you end up with? Sex in cold, salty water; slowed breathing (due to air tank and mouthpiece), occasional inhaling of said cold salty water, etc. It sounds like a bad experience for all involved.
At best, you could have sex in a warm bathtub using snorkels or something, but to me that just seems needless as I'm doubtful of any real improvement in having sex underwater as oppose to on land.
Someone once asked me why I call myself Unimportant. Simple, I said. As a nihilist, that's what I see everyone and everything as. In the end, we are all Unimportant.