Most people like to arrive early to get the swim out of the way, then relax the rest of the day.
The other reason you want to arrive early, is a long line forms at the snorkle station. Yes, they only let 1000 people in, but if 1000 people are trying to get their snorkle gear at the same time, you'll end up waiting. After that, everyone needs to grab a wet suit, and the medium sizes go fast. So there are many advantages to arriving early.
The water temperature is the same no matter when you go, it's cold. Take a shower with the dial turned to cold, and that's what it will feel like.