Your last photo looks like the one in this picture of the Florida Cottonmouth, particularly with the diamond "bullseye" pattern. From what I read I sure hope you were using a telephoto lens to snap those pictures! I don't have any personal experience with them but I wouldn't be chasing it. (I chased a Bull Shark once on a dive in the Caymans to get a good photo and didn't learn until afterwards that it's one of the more aggressive varieties. Never did it again.)