Have you heard of the Thompson Motif Index? It's a huge 6 volume index of folklore from various cultures, organized by motif. It's also available for free online.
Looking at the index, there are a lot of entries for "water spirit as woman." I've listed the entires below:
F420.1.2. †F420.1.2. Water-spirit as woman (water-nymph, water-nix). (Cf. †F423.1.) *Type 316; *BP III 322; *Fb “nøkke” II 725ab.--England, Ireland, Wales: Baughman, Ireland: Beal VII 11; Norse: MacCulloch Eddic 261; Finnish-Swedish: Wessman 55 Nos. 468--469; Finnish: Aarne FFC XXXIII 44 No. 58; Germanic: Meyer Germanen 199ff., 202ff., De la Saussaye 323; Slavic: Máchal 254f., 271f.; Estonian: Aarne FFC XXV 126 No. 58; Livonian: Loorits FFC LXVI 42 No. 30; Armenian: Ananikian 84; Japanese: Ikeda.
F4126.96.36.199. †F4188.8.131.52. Water-maidens are of unusual beauty. German: Ebermann Elbsagen 115 (No. 66), Meier I 67 (No. 1); French: Sébillot France II 196, 345; Icelandic: Boberg.
However, there are also a lot of entries for "Water-spirit as man." For example:
F420.1.1. †F420.1.1. Water-spirit as man. Takelau (New Zealand):
Beckwith Myth 150.
F4184.108.40.206. †F4220.127.116.11. Water-spirit appears as handsome man. German:
Ebermann Elbsagen 93 (No. 55); French: Sébillot France II 409;
Finnish-Swedish: Wessman 57 No. 487.
F418.104.22.168. †F422.214.171.124. Water-spirit appears as black man. German:
Knoop 89 (No. 145), Sommert 108.
F4126.96.36.199. †F4188.8.131.52. Water-spirit as small gray man. Has green
eyes and gray hair. Tobler 99.
F4184.108.40.206. †F4220.127.116.11. Water-spirit as man clothed in white and
accompanied by a dozen attendants. Chinese: Werner 182.
Finally, there are equally as many entries for "Water-spirit as animal". There are actually too many examples for me to quote in this answer, but some examples are:
F418.104.22.168. †F422.214.171.124. Water-spirit as fish. Tobler 97; Icelandic: Boberg, German: Henne-Am Rhyn 113 (No. 216), Ebermann Elbsagen 74 (No. 39); French: Sébillot France II 196.
F4126.96.36.199. †F4188.8.131.52. Water-spirit as horse. England, Scotland, Ireland: Baughman; Icelandic: Boberg; Danish: Kristensen Danske Sagn (1893) 163ff., (1928) 112ff.; Norwegian: Solheim Register 17; Swedish: Hartmann 27; German: Künzig Badische 31 (No. 87), Karstens Sagen 79.
Finally, there is also "water spirit in abnormal form." Again, there are too many entries for me to list here, but a few examples are...
F4184.108.40.206. †F4220.127.116.11. Water-spirits with human body and webbed feet and hands. Tobler 98; German: Henne-Am Rhyn 113, Pröhle Deutsche Sagen 153 (No. 119); French: Sébillot France II 403.
F418.104.22.168. †F422.214.171.124. Water-spirits with green teeth. England: Baughman; German: Schöppner I 219--223, Schultze-Gallara 23; French: Sébillot France II 343 (green eyes).
So no, I wouldn't say that there is a "preponderance of seductive water nymphs in mythology." Women might be slightly more popular, but men, animals, and "abnormal forms" are also significantly represented.