Night dives are fun and you usually get to see things at night you don’t during the day since a lot of sea life comes out and hunts food when it gets dark. And as LA said above, you normally splash before it gets completely dark so you have a gradual darkening to get used to it.
I was apprehensive before my first night dive, which was done in Cozumel. It turned out to be one of the most relaxing and enjoyable dives I’ve ever done.