Like you, I LOVE Nachi Cocum so after seeing your post about Almaplena, I pulled the trigger and booked it. I know you were concerned about getting to the taxi, etc so I wanted to share info from the email I rec'd from them this morning :)
Transportation:
Please travel together in a regular Taxi up to 4 people (if you would take a different trasportation additional rate will be at your charge).
Round trip Taxi transportation is included (from and to Costa Maya Port), it's fine to take transportation inside or outside the Port.
At your arrival please exit the Costa Maya Port, keep walking straight about 3 min. and on your right or left side you'll see the taxi meeting area.
Please see the attached picture of the taxi site transportation and a map of the port area and outside the port area. The YELLOW taxi cab stop is there on the right hand side approximately where you see the #0. That's where you'll have to go to catch a cab to Almaplena. They know your ride to Almaplena Resort is included. The ride will take about 20 min.