You might never know it from just driving around, but the coast here is backed by a massive crescent-shaped lagoon known as Laguna La Joya, extending well inland from Puerto Arista [1] to past Boca del Cielo [2]. Fringed by low trees and tangled wetlands, it’s home to countless water birds, not to mention a significant amount of the fish caught and sold here.
There are no official tours of Laguna La Joya, but local fishermen occasionally take visitors on fishing or bird-watching tours, and the lagoon itself, a rippling expanse of blue with the jagged spine of the Sierra Madre rising abruptly in the distance, has a striking beauty.
You should be able to find a willing guide along the waterfront in the village of Cabeza de Toro, on the road toward Boca del Cielo [2]. (Coming from Puerto Arista [1], turn in at the large water tower.) A two-or-three-hour outing should cost around US$45; try to pass by in the morning to arrange an afternoon tour.
Links:
[1] http://www.moon.com/destinations/chiapas/the-pacific-coast/puerto-arista
[2] http://www.moon.com/destinations/chiapas/the-pacific-coast/puerto-arista/boca-del-cielo