Posted by Lee M. (63.73.5.88) on December 17, 1999 at 00:42:18:
Main Entry: water dog
Function: noun
Date: 14th century
1 : any of several large American salamanders; especially : any of a genus (Necturus of the family Proteidae) with external gills
2 : a person (as a skilled sailor) who is quite at ease in or on water
I think # 1 Fits here.