I will join Scubachick1976 in being late with this thread.
I have to start by saying that my "home sea" is the Black Sea. When we have 10 meters viz is... great day, amazing!!! My first dive there, barely seeing my hand, was frightened. The fourth one was already bearable. Than, the following ones were enjoyable and so on.
During my AOW course, some years ago, one of the dives was in a cloudy, grey day, in my beautiful Black Sea, at a viz of just 1-2m. On the boat (a small Zodiac) we were 4 students and the DM. As soon as we reached the spot - a maximum 20m depth - the DM jumped in the water and get down to fix the anaemic anchor line because the boat man didn't manage to do from the boat. Anyway, the plan was the DM will go down and wait for us there. We, students, are supposed to dive in two pairs, using the anchor line. The first two (a guy and his wife) gave the signal and disappeared under the waves. Me and my buddy made the final check, gave the signal and start the descent. Everything was ok for the first couple of meters, than suddenly my buddy jumped out to the surface. I followed him and asked what's the problem. told me something about equalizing trouble and gave the sign for going down once again. After three meters, same story. He ascended, I followed. At the surface, he said he had some problems with his regulator and he needs to call the dive. He jumped in the boat. I spoke with the boat man and decided I go down and join the group. For the records, I always had a kind of issue with dark waters. You know that nice night swims in the sea? A group of boys and girls running in the dark waters and swim under the beautiful moonlight? Well, even if somebody would offer me a bag full of cash in exchange of just 5 minutes night swim I wouldn't wet my ankles. You can go and laugh on me but... that's it. I was afraid of those dark waters. Now, coming back to that dive. I decided I have to face my fears. I grab the line and dove. When I reached 6 meters, I was swallowed by dark. At 8 meters was absolutely pitch dark. I had a light but I decided not to switch it on, thinking this way I would have greater chances to see the DM's light. All the terrifying characters of horror movies joined me. I started to think about calling the dive but suddenly a faint light flickered down there. When I reached 18 meters, I was more than released to join the DM and the other 2 students. We took "a 20 minutes walk" in the dark, an absolutely night dive performed at noon. Everything went Ok. After that, in preaty similar conditions (a night dive in the middle of the day), I had no absolutely no problem, even if I had to take care of two just new certified divers.
My point is that going in a strange, different environment will give you a certain level of stress. Doing things step by step will increase your confidence and help you to overcome your fears.
BDW: now, the guy who was afraid to swim in the sea at night is a cave diving student