I've seen what appears to be two sea snakes washed up on the beach - in Teacapan a few years ago and again a few days ago in Mazatlan. From what I can tell online these are way out of their customary ...
I've seen several yellow-bellied sea snakes on the beach in La Manzanilla, Jalisco. They usually are not common on the shore, but have probably been injured and washed ashore. In 2 cases I had to warn children away from the still alive snakes who were using sticks to play with them. They can be very poisonous.