I'm scared of fish! They gross me out. I think it's because they're squishy or something. I generally try to avoid them - I hate following a line in the river out to a wreck because the lines always go through tall weeds underwater and I'm always worried there's a giant fish waiting to attack. Attack pike!
I want to try some reef diving this winter to see how it goes. I know it's a ridiculous thing to be scared of - there's always a chance she agrees it's a stupid fear. Fish up here are ugly too... I'm okay with teeny fish, but when there are schools of fish, big fish or eels I don't like to get too close. One of the reasons u/w photography doesn't interest me (although I prefer to blame it on the price of gear and not my fear of getting close to fish).
My dive buddy/instructor had a good laugh one day - well, more of a head shake - I was out diving with my brand new regs and we were "treasure hunting", just kind of seeing what was out there - and I went to reach my hand forward and almost touched a HUGE fish hiding in the grass. I screeched through my reg and zipped over to my buddy as fast as I could to get away from the fish in case it attacked or something. He looked at me, thinking something was wrong, so I conveyed with hand signals the true problem.
For me, I'm just out there diving - but I really want to dive and I'm willing to work through my weird fish phobia in order to do it. My motivation to get underwater and see wrecks and go exploring is strong enough that I'm willing to put up with the bad stuff (fish) in order to do it. Maybe the key is to appeal to other parts of diving that are interesting? Wrecks, neat formations, underwater plants... etc.? Maybe she'll just consider the fish an unpleasant side effect.