How about air motors that discharge into a hose long enough to reach the surface or at least discharge far enough away that the noise isn't so significant.
I don't know...
Scuba discharges bubbles in frequencies that we can't even begin to comprehend...
And since water carries sound 8x faster and about 200 times farther than air does, I'm not sure how we would go about getting the air discharge far enough away...
And at depths between 100 and 300 feet (Nitrox), a line to the surface would be problematic at best...
I'm thinking Gel Cell battery in an insulating gel so compression/decompression won't be an issue,
Neither will (salt) water intrusion.
Stuff the batteries/electronics in a small, fitted compartment and pump it full of something like dielectric grease,
Then all you would have to worry about was the electric motor seals...
The old 'Trolling Motor' trick only works down to about 60 feet or so, then the seals give up at about the 3 atmosphere mark...
(Ask me how I know that!
I've got some pretty powerful DC motors, but I'll have to build some elaborate seals for the shafts since you can't liquid fill a drive motor and expect it to work!
Still, I see the guys with the air motors,
Nothing more than scuba air for power,
Never have to do any more maintinance than a squirt of lubricant into the lines once you come up and blow them out,
And they LIVE FOREVER!
Light weight, powerful for weight, and compact package!
Rotary motors look like A/C compressors inside, and since they are made of resin plastics, they are CHEAP!
If I could just beat that noise!...
Defuser maybe? Like a 'Muffler' under water?
'Sock' with a bunch of tiny holes?