I swim almost everyday with garden variety tinnitus, if there is such a thing. All the doctors I have asked telling them I have rather chronic tinnitus is, swimming will do no harm to tinnitus and as a stress reliever it is a net positive.
I was a competitive swimmer growing up. Swimming is a big part of my life and have taught it as well and still do at a reduced level. As to being underwater, I routinely swim a fair amount under water and in fact have competitions with other people for fun as to who can go the farthest.
I will say this about tinnitus and swimming and OP I can't speak to any particular disfunction you may have like ETD.
Swimming is one of the best medicines in life and when I am shredding the water, my tinnitus melts away. I never hear it. Water rushing over my ears completely dissolves it.
Now a wacky element. When I get out of the pool I can hear my tinnitus again. Many times my ears are plugged from the water. And I will jump on one leg with head tilted to knock it out. I notice a couple of years ago one particular side when I do this. Most of my high frequency hearing loss and highest volume tinnitus is on my left side. When I jump up and down on my left leg with head tilted to the left to knock the water out of my left ear, my tinnitus flares with every jump on my left side. It sounds like a tambourine or...the same high frequency pitch as my tinnitus only louder.
This of course is some kind of somatic influence my tinnitus has. It has to be related to some kind of pressure I have inside my head that becomes momentarily increased by jumping on my left side with head tilted down. I do not hear the same thing on my right side. But, I don't have much tinnitus on the right side.
My thoughts.
OP, you maybe overthinking it but I honestly don't like that term. Can't overthink stuff. You think about stuff until you are comfortable with your conclusion. We each are different There maybe an interaction with water and your ETD for example. I would listen to anecdotal accounts here and if you want further conclusion, speak to your doctor or doctors about it.
Good luck.