I have the same fear. I had taken swimming lessons at 5 years old. They wanted us to jump off the diving board. If we were hesitant, then they would say that the dragon would come up from the drain. I know that this is ridiculous, but that's my fear. No matter what size drain it is, I'm very terrified. If I can't see the drain, I'm better off, but I always look for where the drain is when I get into an unknown pool. I am an avid swimmer and was a diver growing up. It was very hard for me to dive with this fear. I would do my trick and quickly swim to the side. One time after a dive, I happened to touch the drain. OMG! I was so scared! I never overcame it. I'm 56 years old and still terrified. I'm getting better with the underwater lights but I never had a fear of the filters though. I never went to a therapy for it.