Restaurants on the Coast
I have eaten some great fish along the Los Tuxtlas coast.

Unfortunately, there are very few restaurants along the Gulf that are reliably open outside of the few
weeks of the over subscribed tourist seasons.

Palapa style "dining" is available in dozens of palm frond stands during the few weeks of the seasons.
Then the wind blows them away.

Reliably open and recommendable is
La Casita in Dos de Abril across from Montepio. In La Barra, La
Sirenita
makes an effort, and in Costa de Oro, the tiny restaurant La Carmelita will grab a fish from the
shore-fishing nets to feed you.

There are several restaurants around the embarcadero in Sontecomapan not worth mentioning, except for
the one above the Poza de los Enanos, which serves an excellent
Caldo de Marisco, (seafood soup),  
unfortunately frequently spoiled by a very noisy jukebox.
La Sirenita, La Barra, Catemaco
Antojitos Carmelita, Costa de Oro
La Barra
La Sirenita, La Barra
Sontecomapan