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 ...
Diving in waters around GUAM we came across them often.
YES they can be agressive when it's mating time They are also very curious which can make them seem agressive cause they will 'check' you out
Yes they are very poisonous! Fortunately they do not bite in 'defense' so bites to humans are usually 'blanks' They will bite your flippers as you back pedal away.
Inhabit shallow waters so they tend to get washed up on shore following a storm