We go to CN in February, usually the week after Valentines day. It is peak season then, I think they consider January - March as peak, and although the resort is at capacity, it never seems really that busy to me. May wait in line a few minutes to get seated one of the restaurants, but nothing out of the ordinary or long. I am always amazed they are full. Can't speak for the other Couple resorts.
We were at CSS in early March 2013. The resort was filled to capacity. My sister couldnt book ANY room to attend our wedding. Each and every day we felt like the only people there. We always got our snorkeling times. Never had a problem finding a lounger. We were able to get reservations for dinner each night we requested. And I think maybe twice we had to wait for one or two couples to be seated in front of us. And by waited I mean like less than 2 minutes. It never felt like it was filled to capacity. Not once. I dont know how Couples does it, they just do. Dont worry about it being over crowded, just enjoy your vacation and each other!
We just got back last week, and this is wedding season. There was about 4 weddings that we could see but CSA is pretty big and we didn't feel crowd at all. We actually thought there might have been less people at the resort this time then when we were there in july 2011.