I have been snorkelling in Grand Cayman a few times and yes the water will be warm even in January and the snorkelling is very good. Whilst there you will probably be taken to snorkel over "The Wall" which is a weird experience as the sea bed suddenly drops from about 10/15' to total blackness down to I think it was one and a half miles deep, just a sheer straight drop. I always take my own fins, mask and snorkel and was able to find several great spots to go just on the beach in George Town. Just make sure that you take advice and go in pairs but I needn't tell you that need I?
Thats great, and yes we always take our own kit too. We swim together so we can point things out to each other. Hope its even better than Mars'alam, Red Sea, was in March this year. Thanks, Neil.
I have also snorkelled in Grand Cayman. You should also visit stingray city. Here you can swim with the rays.