I found this thread by doing a Google search for "pool light screw stuck." I read through the responses and figured that my options were either to (A) drain the pool and use power tools in hopes of getting the screw off, (B) use the SpeedOut to try and get the phillips head screw unstuck, or (C) try using a hammer to carve a new groove and then hope that I could get it unstuck.
I did get the screw out. I didn't use any of the above techniques. Here's how I did it. I can't say this will work for everyone. Your mileage may vary.
I took a small 1/8" flathead screwdriver like this one:
1/8-Inch Flat Head Screwdriver with Cabinet Tip, 3-Inch Round Shank and Comfordome Handle Klein Tools A216-3 - - Amazon.com
www.amazon.com
It had the hard plastic on the back. The tip was wide (wider than those small ones you use to fix a watch or computer parts) but it was also thin at the head. Almost like a knife. (If I didn't have this, I'd use a sharp knife to start.)
Using a hammer, I treated the screwdriver like a wedge and was able to pry a small gap between the top of the screw and the metal ring. This wasn't particularly hard but I was doing this under water so was limited to 5 to 10 good hits before having to resurface for air.
Once I had the head of the screwdriver under the screw, I tried to pry it a bit and *snap* off went the tip of the screwdriver. Shoot. I had a second one, so I switched to that. Snap. Same thing. So keep in mind. If your screw is really stuck, you may break a few tools doing this.
Once I had a gap established, I used a larger, more robust flathead screwdriver (that was less likely to break) and again, using it as a wedge, hammered to widen the gap between the screw head and the metal pool light ring. I continued working around the screw trying to get an even gap.
I'm not going to lie. This was not an easy process. But I could see progress being made. I could see a gap forming all the way around the screw, and that was a whole lot more than I'd had before.
I continued using subsequently larger, fatter, bigger flathead screwdrivers and hammering all around the screw to form a gap between it and the metal pool light ring.
At some point, once the screw was slightly separated with a gap all around it, I tried pulling the pool light from the wall. It was stuck tight. I couldn't tell if it was because of the screw, or that it had just been years of it being in the same place. So I used a thin screwdriver and tried prying it gently away from the wall. After a bit, I was able to get it a little loose. That's when i realized that this was going to work.
With the metal pool light now wiggling a little bit, I knew that I just needed to get the screw far enough out. Then I could either saw it off, rip it out, etc. I really wasn't sure. I just knew I was making progress.
I continued tapping with larger flathead screwdrivers and the gap all around was about 1/16" so I started prying back and forth to get the head to bend a bit out. That didn't really work, but it did widen the gap just a bit. ]
At this point, I started using pliers to see if I could grab the end of the screw. And I could! Using a heavy duty set of pliers, I was able to grab the tip of the screw and turn it. I can't even tell you how excited I was to see that screw finally turn.
I used the pliers to turn the screw until it came out.
All told, the process took about an hour and a half.
So in summary, I used a hammer along with small screwdrivers which acted as a wedge. I worked all around the head of the screw and was able to separate it from the metal light ring. Once I had enough of a gap (which did take over an hour), I was able to grab the screw head with a pair of pliers and from there, I was able to unscrew it.
I hope this helps somebody else!