It can't be a leucistic as it is not white. In fact from the color/cloudiness of the eyes in at least one photo, it looks like it's in shed.
In spite of the location I am inclined to lean towards a species of water snake, though it could be a melanised garter. It is odd because the color pattern is more like a black rat, but the head looks wrong. Give it a few days and see if it does shed, then re-photograph it.
If you find another example show us that too please as It will establish "norms".
Frank
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"The luxury of not getting involved departed with the last lifeboat Skipper..."