Fishman- The exact location is at the far south end of the harbor- look for the big PEMEX sign near the big sunset cruise boats.
It would be nice if they were open dawn until dusk.
So at a 10% premium in the fuel price, It would cost me about 50 bucks more per tank (150 gal), so the only way it is cheaper for me to use the fuel dock is if I need a small quantity of gas to top off the boat. Otherwise, it is still cheaper to pull the boat, gas it down the street at the Pemex on Sinaloa, and then pay the Marina guys to re-launch.