Start with one if you are just a beginner, as you will have enough to worry about. Also starting with one will make you think more about your lighting, the best position for the strobe for each shot. I have seen plenty of beginners using two strobes (because they see images of all the pros with two) and then not move them once during a dive and wondering why they get flat, washed out or images full of backscatter.
Keep it simple and have a lot of fun.
- kc_moses likes this