tl/dr: You will get wet, and there is almost 100% chance you will see turtles and sharks (even if it is just from a distance) but don't expect to be blown away.
It looks like our first day was a bit unusual - from then on we had 20+ divers on the boat. We've used this board for research for our dive trips, so in case someone is looking into beaches, here is a run down of how things were for us.
We were able to sign up for the full week of diving on the first day - but I think when it is busier you need to sign up each day. Each morning you need to check in before the dive. Unfortunately at the 8:15 checkin time there are also people checking in for snorkeling and glass bottom boats, and one tank dives and discover scuba diving, so it is a bit chaotic until people figure out they need to make separate lines for each.
You need to do a resort orientation - if you have dove recently and have a certain amount of logged dives it is just a verbal briefing. If you haven't done any dives recently, or haven't done many dives you need to do an in pool orientation, and possibly a refresher course. One guy on our boat did the verbal orientation @8:15 and was able to dive that day (the boat left a bit late) but again, I don't think that is normal.
We did the two tank dives, and everyone on our boat each day was a certified diver. They split the boat up into 3 different groups based on experience and cert level. We are advanced, and so we were always diving with advanced divers.
The first dive of the day would be ~45 minutes away from the resort at one of the Northwest Point dive sites, and would be a deeper dive. We spent the surface interval coming back closer to the resort, and the second dive would be at one of the sites in Grace Bay.
The diving itself was DM lead dives. Most people didn't really buddy up, and if they did, the buddy groups didn't really stay together very well - and everyone just followed the DM. You were definitely not encourage to drop in and dive as a buddy team and find the boat on your own. Turn around is when someone hits 1500psi. Most of our dives ended up being in the 45-50 minute range.
We thought the diving itself was just okay - which is kind of strange. We generally saw a couple of sharks on the first dive, and a couple of turtles on the second dive, and groupers on both, but other than that (which are definitely highlights for most divers) I think the coral and fish pale compared to the other places we have dove (Bonaire, Cozumel, CocoView in Roatan, Lighthouse Reef in Belize).
Also, the divers we dove with were very much "Vacation divers". Some just went on one day of diving during their week here, and many were certified at Beaches and do an annual trip there, and have never dove anywhere else. We talked to some people who said the diving here was really good, but their only other diving was at Beaches in Jamaica. A few brought their own mask and snorkels, but we were the only ones I saw all week who brought our own gear, and we were also the only ones who signed up for 5 days of two tanks in a row, although we did see some familiar faces each day.