I dove in St Kitts almost 10 years ago, so this may be a little outdated. We went in September, and the weather was decent, water temps around 80, reefs were mediocre and vis was ~50 ft. It was one of my first trips after getting certified, so I really didn't know what to expect. At that time there were only 2 dive operators (Austin McCleod was the DM's name that I dove with). Both had similar operations. Single engine boat, 15 - 30 minute ride to dive sites. Service was good, but not exceptional. I was somewhat put off by the DM catching lobsters on several of the the dives. St. Kitts does not have a marine park to my knowledge, so technically one can catch fish if you choose - even spearfish. Any dive day is a good day, but overall I was disappointed.
Are you going there just for the diving? If so, I would consider other options. On the other hand, St. Kitts is a very pretty island, and I highly recommend doing the climg to the top of Mt. Misery (with a guide).
Hope this helps,
Rich