I live in St Thomas and I'm glad to hear you will be diving with Red Hook. They run a good operation and are friends of mine. As for shore dives, there really aren't many choices other than Coki. You will do a shore dive and see a few bar jacks, sergeant majors and damsel fish mostly. In my opinion, not worth doing and Coki can be a place of opportunity for someone to liberate you from any stuff you leave on the beach. Go snorkeling around the point at Sapphire. Nicer beach, nice fish and sometimes turtles to see over by the point.
Low key and Red Hook dive the same sites. Go over to STJ for a different flavor, take the people ferry and rent a jeep over there. Go out to Skinny Legs or Miss Lucy's in Coral Bay. Both serve a good lunch except Miss Lucy is closed on Mondays. If you are lucky, you will get to see the donkeys on the way out there. Also stop to see the Annaberg Sugar Mill ruins. You can take a short walk to Water Lemon Cay for good snorkeling. I have seen an octopus, ray, and many juvenile fish there. Nice limn' on the beach at Cinnamon or Maho. Good place to have your post-beach drink is Woody's before catching the ferry back.
As for diving, stay on the boat and take a guided tour. They can take you on the best sites like Arches and Tunnels (if the weather is right), Cow Rock/Calf Rock, Ledges and more. You have a better chance at seeing angel fish, trumpet fish, lobster, maybe a crab, a school of bar jacks and other cool stuff. We also have a problem with the invasive Pacific Lionfish. We cull ours here or they will destroy our reefs by eating all of our juvenile fish. They are beautiful and you may have the opportunity to see one.
Water temp is around 80 degrees here right now. Sunny days, bring sunscreen and a hat.