If your tub is Fiberglas try a gel coat cleaner/polish.
I think it's acrylic
Try a mild polish or gel coat cleaner.
I have the 13" and 24" versions... they work OK, but are pretty sharp and rough. Both mine are "Bell Lifestyles" imports.
The 13" is handy to cart around, had a reddot on for a bit, but switched up to a 2.5x Lyman...missed a few too many times with the dot.
How's the Lyman working for you?
I'm going to go with a cheap simmons .22 mag scope