As CamG said, peer pressure and a desire to please someone are well-known risk factors for anxiety and panic in scuba diving. Sometimes nervous or reluctant divers will find some excuse to avoid diving and I would guess that her fear of fish might be the one for her. My wife is certified, but now she says she prefers snorkeling, so that's what she does while I and a buddy dive. You might check out the article I wrote for Alert Diver magazine this past spring, which discusses a case study of panic in a "perfect storm" scenario.
Just this morning after church, the pastor's wife told me she'd like to learn to scuba dive like her son, but she is afraid of sharks. I told her that she was not on their menu and offered to help. We are in North Carolina, so I reminded her that doubtless many sharks have swam near her on the beach and she never even knew it.
The standard approach to dealing with any phobia, even icthyphobia, is prolonged exposure and response prevention. In other words, expose the person to the feared object or situation for 90 to 120 minutes without letting them engage in avoidance behaviors. (Yes, I know, most people can't make a standard tank last that long, but it doesn't have to be in deep water.) Often it is helpful to teach them relaxation techniques like diaphramatic or belly breathing ahead of time. FREE training MP3 files are available from
www.DivePsych.com for just such a diver, even "nervous Nellies".
Since 2000 I have worked with a number of both anxious students and traumatized certified divers, including very experienced instructors, who have been able to get back under the water to enjoy God's creation. I would be glad to talk to her at her convenience. I will also be at DEMA Show in Orlando next month at the NeilMed Pharm Sinus Rinse booth if anyone wants to come by and chat.