I heard a lot of concern over snakes in the water before I went to PV. WE went snorkelling off the beach (my wife and I) and saw what at the time we thought was a sea snake. Partially because it looked and moved very much like a snake but probably also because we read the reviews. After developing the pictures I tried to identify the snake but I could only match it up as a "spotted snake eel" , "Pacific snake eel" or a "tiger snake eel".
I couldn't find too many spotted sea snakes (although I'm sure they do exist).
Also worth stating in my web search, yes a sea snake can bite you (apparently the small mouth thing is a myth) , however they aren't likely to do so unless provoked (ie hands off and you'll be ok)