A couple are looking for sea snakes. The wife has an affair with the boat captain in a pool. He then black Mails her into more sex the boat captain is killed when he is bitten by one of the snakes

Sounds like "Dark Tide" (1994):

Dark Tide (1994) - IMDb (http://www.imdb.com/title/tt0106665/)

It's been years since I've seen this movie. Hope this is the correct one. :)