I haven't ever caught a Mangrove but I have caught several Gulf Salt Marsh Watersnakes (N. c. clarkii). They also spend a considerable amount of time in salt water. None of the individuals I've caught have had any blisters. As a matter of fact, they're the cleanest looking species I've found. All have had complete tails and no scars. I have caught several Brown Watersnakes and a couple Banded Watersnakes with blisters.