There are 54 Pokemon that are pure Water-type, and 57 dual Water-types.
However, it should be noted that Marill, who was once a pure Water-type is now Water-Fairy, so if you don't include Gen VI results, it's 55 Pokemon that are pure Water-type, and 56 dual Water-types.
Also, there are two confirmed Water types in 6th gen that are unknown to have a dual-type.
Gen V brought the arrival of rather few Pokemon, so many that it would be easy to list (which I won't do. :3)
Hope I helped. :)
Mr. Source and Mrs. Source