It might help to take your time on the surface to fully relax - take a few slow, deep breaths of your reg before you submerge (that serves two purposes - the slow breaths help you relax and the fact it is off your reg mentally reinforces the fact that your reg is working properly).Im new here and this is my first post but I can relate to your question. I have been extremely nervous about the ocean in general for as long as I can remember and even phobic at times. I got marries in Jamaica back in 2003 and went snorkeling only to jump back in the boat as soon as I saw my first fish and a giant Brain Coral below me. For whatever reason I freaked out and had a panic attack.
Fast forward to last year I tried snorkeling again in the Florida Keys and had a similar reaction but worked through it and was able to relax and spent an hour getting bee lined by barracuda and thought it was really cool!! I have a trip to Aruba planned for August and have decided that if I didnt get a dive or 2 while there it would be a wasted opportunity. So I started my road to certification and just completed my confined water a few days ago and already planning my Open Water and AOW when I have completed those. I guess the moral here is what your feeling is normal but also understand that you can control your reactions to stress and even use it as a motivator.
First time under water with respirator in 4 ft water I started hyperventilating... Then I looked at others and realized they were drowning so why would I? After that I just focused on breathing with eyes closed for about 15 seconds and now Im hooked! My Instructors didn't even realize how uncomfortable I was until I told them later that day. YOU CAN DO THE SAME Understand your nervousness is normal and that your in control of your reactions to it.
It is ok to be nervous (breathing underwater is not a natural thing for human beings) but the nervousness will ease over time. If worried/nervous, have a chat with your instructor/buddy and get them to take things a bit slower.