I still dive with a little sense of fear. I think it helps keep me alert.
The first thing to do under water... is stay calm and keep breathing.
On my scuba 3rd trip i took my wife for her OW. she had 3 pool sessions just to build confidence. When at the island, shore dives were OK, everything went well until we got to boat dive at a site with 40ft depth. I swam away with the leisure group. 50mins later when I got back to the boat, she was throwing up couple of times. she hyperventilated and had partially fainted underwater. the DM assisting the Instructor made the rescue and surfaced her. i learned that she panicked, felt cloustrophobic, then hyperventilated. it was a beautiful 100ft viz that day, and she could see the boat. she thought she could just breach and gasp for air. The instructor didn't pass her. until today i'm still trying to re-built her courage. all that outdoor, hiking, mountaineering, taekwondo, could not prepare her for being underwater.
What i learn during my OW days and i still keep today, is to remain calm, know your equipment, and trust your buddy. know everybody in your diving group. Learn about the site and get as much info from the DM. Observe the landscape, positions of the wall, bouy line, anything. Go down slowly, remember to equalise. stay in group. Eventually you'll get the hang of it. its like riding a bike, or playing a piano. you cannot unlearn it. but beware... scuba diving... its addictive!
Hope u have a great and fun trip.