Stuck screw on underwater light fitting

[Eddles]

Hi all,

My parents pool have a pair of Certikin PU9 underwater lights. The bulb in both lights were blown, so I replaced the bulb in one - no problem, everything came apart fine and put back together. I then tackled the other fitting - I found the top screw to be quite hard to remove but managed to remove it. However, it seemed that the last person to replace the bulb was stronger than Hercules and I couldn't unscrew it. I used the screw I removed to find the exact right tool and it was a pozidriv size 3 and I have a pozidriv size 3 screwdriver so I used that. However, it kept camming out as I didn't have enough forward force to keep the driver in the screw head as I had to be in the pool itself - the fitting is too far underwater to reach from the surface.

The screw head is slightly damaged but I think it's fine. I don't want to go as far as to drill it out as I don't have a compressor, so I'm hoping someone would have a bright idea I can use to remove the screw please?

Many thanks in advance!