Howdy Jay!
First from what I have been reading, the water temps have been around 80 or so. Not too bad but a few degrees warmer would be nice. Do take some sort of jacket or boat-coat because while the water temps may be OK it can be a little chilly between dives.
Typically when we go, we each have a 50lb bag, a carry-on, and a personal item. I don't remember the exact weight you are allowed on your carry-on. I do know that when you leave to come home, they typically weigh all your pieces of luggage. I think maybe in all the trips I have taken there, I have seen them charge one person for being overweight on their luggage. I think the weighing of the luggage has a lot to do with the fact that the plane you fly to Little Cayman in is a 16/18 seater and the amount of luggage can be an issue if some folks are carrying a lot of extra stuff. But if you are talking about a total of three 50lb bags, I suspect you may be charged. I can tell you that I can typically pack all my dive gear and clothes in one suitcase and stay under 50 lbs when going to LC and still have too many clothes.
Not sure what flight you will be on from GC to LC. If it's that last flight of the day, typically around 5:00 or so, luggage could be an issue although typically it's not. On one trip where we were on that last flight, my bag did not make it over on the plane. It was, however, on the very first flight the next morning and all was well. Again, because of weight issues and a limited amount of luggage/weight the plane can hold, just be prepared. It seems like a big deal at the time if it's your luggage not making it but all was well in the end and no dives were missed. Now if you are on an earlier flight in the day which typically tend to be less full, you may be just fine.
Not sure where you are staying but I hope you have a great trip. The diving there is as good as you get in the Caribbean.