Hi Mina - this topic might be better posted in the Tulum forum, but I have been so I can try and explain and if you have any specific questions feel free to ask.
OK - Bahia Principe Coba - is a part of a whole Bahia complex - the oldest past is Bahia Principe Tulum, then there is Bahia Principe Akumal - a little more upscale and now there is Coba. They were building these units while we were there. I think I am correct in saying that the Coba and the Akumal side share all of the same facilites as far as dining, pools, etc,,,,,, Also I believe these guests can use all facilities at the Tulm side, but Tulum can not use the ala cartes on the Akumal Coba side.
The complex is HUGE!!!!! There are trains that take you throughout the resort which we used, its not an issue but be aware that you will wither walk alot or be prepared to wait 5 10 minutes for the Tram to take you to your rooms, etc... Between the 2 resorts at the entrance is the Hacienda area where they have shopping, a few restaurants and bars, its pretty fun there, and I beieve Fridays is their big fiesta night there. You take a trolly out to it.
Overall it is a great resort. Downside is it size. The food and staff are all very good, noting exceptional but all very nice. The beach is somewhat rocky, but there is a lagoon area between the resorts where most people do their swimming and its nice and cal min there for the kids to play in the sand,
Hopefully this has helped you and Im sure youll find more info on the Tulum board if you look there - good luck.