I see you've already booked, but for others interested I would also recommend Ocean Frontiers on the East End. It would be a pretty long drive from Seven Mile Beach, but we stayed at Turtle Nest Inn in Bodden Town. It was probably about a 20 minute drive for us. Our original plan was to do several boat dives and a few shore dives, but we ended up just doing one shore dive at Sunset House to see the mermaid. If we go again (which I hope to) I would book a few more boat dives with Ocean Frontiers and leave our gear with them the entire stay. The "dive valet" service is awesome when you are on vacation. At the end of the dive you just put your stuff in the crate they have for you on the boat. They lug it up to the dive locker, rinse and hang it to dry, and it is locked up and monitored by video surveillance. When you arrive for your next dive your BC and regs are already on your tanks and the rest of your stuff is in your crate. If you prefer to set up your own I'm sure you just have to tell them and they'd just leave your gear in the crate. We opted to let them setup and double check everything.
The dive masters were great, especially Elliott (they call him Hobbit) on our first dive. They basically lead you around for a set period of time (around 30 minutes for deep dives and 40 for shallow) then as long as you have a computer you are encouraged to extend your bottom time. At the end of the guided dive they make sure everyone sees the boat or mooring pin then they head up. You also have the option to do the dives unguided in buddy pairs. The entire staff on the boat was also extremely helpful as I easily get seasick. They were quick to ask if I needed water, ginger biscuit, etc several times. Luckily I only got sick on 1 day of diving.
There was one dive later in the week where the dive master was also doing a fish ID course with 2 students and he spent more time trying to get them to identify specific fish than trying to find bigger more exciting things for the group, so we were a little bit more on our own. We still had a good dive though despite this.
You can also book a package and stay at their hotel (Compass Point). It still seems more affordable to me to stay at Turtle Nest Inn, but it just depends on how close you want to be to the dive boat vs Georgetown and the West End of the island. The prices are pretty close.