Chuck and Robbie's dock, if you get in good with them. Eric will sell you some of his catch.
I usually hit the docks in the "embarcadero" just to the immediate East of the old football (Costa Maya grounds) field off of back street. (North East corner). The fishermen usually pull in between 8 and 9 with their catch. Good stuff!
Shaba, vacation club barteneder, will hook you up too. Let him know what you want and he'll get it for ya. Good guy, good resort, good vacation club!