My partner and I booked the Anguilla tour this year. (1/17/20) After having done the Three Island tour a few years ago, we knew we wanted to go back to Captain Alan!
I recommend booking the transport to/from the trip via their transport partner. It made our day easy/smooth.
When you arrive, you're checked in and fitted for fins. From there, the captain, first mate, and crew are introduced. Then you board the boat and after a clear safety demo you're on your way! Word to the wise: If you're sitting in the front of the boat, you'll have the best view, but you may also get soaked.
The Anguilla tour stops at various locations around the island. You stop at incredible beaches, islands, coves, and caves. The snorkeling visibility was really excellent, but if it wasn't I would have been just as happy to relax on the boat with a rum punch and enjoy the incredible views. Go with the expectation of stellar views and a great tour guide, but know that the snorkeling will vary from island to island/day to day, as the charter company can't perfectly predict the turbidity/quality of the water at the snorkeling spots from their marina 30km away.
We saw sea turtles, gorgeous fish, new coral growth. I definitely plan on booking with them for our next trip.