A very popular activity but very ILLEGAL are the swimming with turtle tours that are offered from touts on the beach. These should be avoided as marine turtles are endangered species that are making a very difficult comeback.
On the tours a person jumps in the water and grabs a marine turtle for the boat customers to play with. What people don't know is that human contact transfers diseases to the turtles and most importantly the tourists drown the turtles. Turtles often have no time to catch their breath and start to go down underwater to avoid human contact. They then stay at the bottom waiting for the boat to leave and die.
It is very illegal to touch and play with turtles. Try to respect the law and protect the very species you have come to visit. A very good alternative to Turtle tours can be found at La Ventanilla only 2 kms north of MAzunte. Here the community runs a conservation project where baby turtle hatchlings are released back to the ocean everyday in season around 5pm. You can visit also their Crocodile sancturary for only 4usd. They also offer horseback riding at sunset on their 25km long beach. Visit La Ventanilla it's a hundred times better than anything the touts offer on the beaches of MAzunte or Zipolote beaches.