I'm the one you want to talk to. I have a fish phobia and I recently became OW certified.
I don't know where my fish phobia came from, but it's real. When I got a fresh water aquarium 10 years ago, I couldn't put my hand in the water with the guppies and neons to straighten the decoration. Murky water lakes were terrifying because I couldn't see the fish waiting to bite me. I couldn't put my hand in the touch tank at Sea World to touch the rays. I was snorkeling in the Caribbean once, and had to stop when the fish kept swimming around me. Phobic enough for ya?
Still, I wanted to dive, took the classroom and pool, and did the open water training in the Keys. As a part of my classroom training, I asked questions about fish behavior to educate myself. As I learned more about the normal behavior of sea life, I began to understand and respect underwater life. I think that knowledge is power. It's the lack of knowledge that causes fear.
From the start of the open water training, I discovered that I was not afraid of the fish at all. In retrospect, it felt as if I was behind the glass at an aquarium (behind the mask). I felt 'protected' by the wetsuit and gloves. I did not wear a hood in the warm water, and felt no danger, even though my head was 'exposed.'
Just to let you know, on my final check-out dive, I came upon an 8' nurse shark. I watched from a respectful (10') distance, but felt no phobic fear. My heart was pumping faster due to excitement, not fear. I also saw a huge sea turtle, a barracuda, and two moray eels. The other fish were also beautiful, the corals stunning, and it was a magnificent experience.
On a scale of 1-10, I'd rate my fish-fear level at 10 before diving. After my dive experience, I don't feel the irrational fear any more. (Although, I will never be a fisherman, because I won't hold a fish to remove a hook from the mouth. That still creeps me out.)
OTOH, when the others talked about underwater fear and stress reactions, they are very real. If you become out-of-control-stressed, you could put yourself and your buddy in danger. Be honest with yourself and assess your fears in a rational manner.
Good luck. I hope this is a successful adventure for you.