I dive at least once a week with Red Sail and usually another dive each week with one of the other great dive ops on the island. Red Sail is great, and I enjoy diving with them as much as the other dive ops. Yes they take out more people, but their boats are much bigger and actually feel more roomy that some of the smaller boats who take out only a max of 8. They break the divers up into small generally same-skill level groups so the actual dive is not much different. They are very professionally run and give good service. So you will get a great dive op whether it is Red Sail or any of the other fine ops.
For Westin guests, Living the Dream has an office almost across the street and is another great, valet-service dive op to consider.