In a palapa (thatched-roof) restaurant right by the lagoon, Xel-Ha serves some of the best fresh fish around. Head about 8km north of Chetumal to the fishing village of Calderitas, where you'll find this classic bayside seafood restaurant. Note that they close early if there aren't any customers, so arrive well before 6pm to be sure they're open.
To get here, head north on Boulevard Bahía to Avenida Yucatán. Or take a 'Calderitas' colectivo across the street from the 2nd-class bus station. You can also cycle here.