You just made a strong case against using air tools underwater!

I've never encountered this on a pool light, but I do have a fondness for old trucks. Rusty nuts, bolts & screws are common. There are a few tricks, but all mine would require working above water.

Assuming you've dropped the water level down, here's some ideas (apply at your comfort level);
Start with a break-free, I'd recommend either PB Blaster (auto store) or make a 1:1 mixture of acetone & automatic transmission fluid. Spray it around the screw as best as you can. Give it a few doses an hour or so apart. (DISCLAIMER - I have no idea what this will do to pool chemistry or your pool finish. The trim ring & glass lens should be unaffected)
Try to turn the screws now.
If they are rusted to the point of falling apart, see how far they will go. If you can find solid material, try the vise-grips. If they are completely rusted, you might luck out and be able to break it apart and get the pieces out.
If the head breaks off, you can slide the light over that and remove it. You might be able to access the screw from the back and work it out that way with vice-grips.
If you break off the screw and leave some in the fitting, my recommendations would include a propane torch or a helicoil.
Good luck!