Hi. We stayed 9 days at Beaches T&C in Dec 2013 and did 13 dives while there. We experienced a lot of similarities to what other have said but I will share my own experience...
You will need to fill out their medical form but you will only need doctor's clearance if you answer and say you have any current or previous conditions (i.e. you suffer motion sickness, you have diabetes, etc..) If you have dove in the last 3 months they will have you do a check out dive in their pool. It doesn't take but 20 min if you are comfortable with your techniques. They mainly want to make sure you can control your buoyancy and remove and replace your mask underwater.
You will want to get your name on the dive manifest as soon as you can for each day. If they aren't busy, they will allow you to put your name in for the week, but if they are busy, they would only allow reservations 2 days in advance. While we were there they were very busy. However, we did not have any issues diving every day that we wanted to.... we just had to remember to register each day for the day-after-next. The boat was usually full but they brought ample DMs so that they could split the group into smaller groups. I especially loved this because they would keep all of the AOWs together so we could go a little deeper and explore a little more than the less experienced. The dive team was great! A lot of the sites are long boat rides so having a good dive team with great personalities is important I think. Danian, Oats, Donkey, Captain Crunch... just a few of the fun and personable guys... They all took the time to get to know you and would recommend the best dive sites for you. They never go to the same place in a 2day window...but truthfully we dove all week and we only went to one site twice. The thing that I liked the most about this dive team was their respect for the environment. They were serious about leaving only bubbles and taking only pictures... they did not tolerate anyone touching the marine life... this was refreshing for me to see having seen so many DMs in the past let you touch anything you want

As far as the sites themselves... I would say they were good. Not great. We did see lots of life... reef shark, nurse shark, rays, eel, flounder, lobster, a few crab, conch, and of course cute little fish ....nothing however in large quantities.
As far as the resort goes... we've been to 4 in the sandals/beaches family (Grande St Lucian, Grande Antigua, Bahamas Royal Bahamian, and Beaches T&C) and this was the biggest and best resort of the 4!! I definitely DO recommend this beaches and would go back.
Go luck!