We used to get to CSA early in the morning when AJ still ran the LAX - MBJ nonstop red eye. We'd head over to Patois Patio for breakfast, hit the beach and say hi to Elvis and all our friends, and have a hummingbird or two at the beach bar. We were always in our room before noon.
Couples will welcome you no matter what time you arrive.
Just go to the towel hut and tell them you just arrived - have no room assignment and they should give you one. Believe me - they will remember you and when you get your room turn back the dirty towel.
Have a nice leisurely lunch (which the welcome folks will suggest), a couple of drinks (Cassava Terrace @ lunch for CN), a couple of drinks @ the beach bar. By then the room will be ready.
Make sure you have access to your sunscreen, grab some lunch then find your way to the AN beach. That way you don't have to worry about finding bathing suits in your luggage.
My wife and I arrived at 11:00am, checked-in, my wife put on her bikini top, we found our way to the beach and the bar, and applied our sunscreen. We went back to the front desk about 4:00pm and picked up our room key. Our luggage and golf bags were waiting for us in our room.
Next visit we won't worry about having access to beach clothes we will hit the AN beach after we have lunch.