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 am not sure if what we see while fishing is a sea snake...it is a snake in the water, so it fits the description. There are a few in this area which is just North of Puerto Vallarta. The scuttlebutt is that they are very poisonous. Probably about 2 feet long and not large around..