The Cañon del Sumidero is 32km long. It's situated on the San Cristobal road some fifteen kilometres from Tuxtla.
My suggestion:
Note that boats only leave when all seats are taken and hold between 10 and 15 passengers, depending on their size.
Summary:
The best way to tour this canyon, which is part of a national park, is by taking a motor boat from either Chiapa or Corzo and cruising along the Grijalva River. You can also get a great view from the roadside watchtowers at La Atalaya, Los Chiapas and El Tepehuaje.