Molasses reef is probably my favorite reef in Key Largo. It's a fairly easy dive, not too deep... a lot of it is less than 30' iirc. However, at times, it can have a pretty strong surge. I think navigation is one of the main things to pay attention to. It's very easy to go from formation to formation and get a bit turned around because it's not a straight-line reef. I make it a point to keep a pretty good idea of the direction of the boat and will usually come up about half-way into the dive at the turn around in order to spot the boat and fix a compass heading to keep me on track to finish near the boat.
Last Sunday, the seas were 5' or so with a pretty strong surge, and we had a lost diver situation come up at the end of the dive because he didn't respect his time or navigation. It was pretty hairy for about 15-20 minutes at the end of the dive when everybody was trying to spot him on the water in a light rain. No whistle, no smb. He was eventually spotted climbing up a tower about 1/4 mile away. Thankfully, he was retrieved safely, and I think quite a few divers had some good lessons reinforced. It did, however, cost anybody interested a chance at diving in the afternoon. :-(