For a long time I've been getting iron stains in the pool and could not figure out where it was coming from. I thought all the steel in contact with water was 316 stainless. Today I noticed the underwater light looked a little dark around the edge. I laid down and felt the ring that holds the fixture against the wall and it is rusted and badly pitted. The rust is not built up like it is mild steel and I could rub it off with my finger so it must be some iron containing alloy.
Is it common to put that kind of retaining ring around an underwater light and can it be replaced while it is submerged?