The Silencers, the 1966 spy comedy with Dean Martin and Stella Stevens, was the film that first got me interested in WAM in mainstream movies. I saw it on TV as a teenager in the 1970s and loved the scene with Stella in the rain and mud (see clip below). I was already a fan of wet hair and the way her hair was saturated by a mix of rain and mud with close up views was brilliant for me, not to mention the whole slapskick element of her coat getting stuck in the door, her feet sinking in the mud before she fell back then forward into the morass.
And at the end there was a second scene (wetlook only) where Stella and Dean are on a bed that tipped forward automatically to dump them both into a sunken bath full of suds. The film ends on a still just after they surface with Stella's carefully-styled hair completely soaked. My teenage fantasy for some time after that was to have a bed and sunken bath like Dean's, sadly one that was never realised.
In those days there were no specialist WAM movies, at least none I knew of, so I had to manage on the more meagre diet of what might turn up in mainstream movies. Although there were some other good ones and it was always a thrill when an unexpected wetlook scene turned up, The Silencers remained my favourite for many years, until Bo Derek came along in Tarzan The Ape Man.
Thanks for the replies. A few other films I enjoyed from that period are Sands of the Kalahari and The Reward, both 1965 and both with rain scenes featuring Susannah York and Yvette Mimieux respectively, Virgin Island (1958), accurately summarized in Splosh magazine as "Virginia Maskell keeps getting wet" and Doctor No, still my favourite Bond film for wet hair.
A couple of stills I found online from The Reward and Virgin Island are attached (despite the b/w images both films are in colour).
EDIT: I recorded the wet scenes from Virgin Island several years ago and I've just checked the recording (not got a lot else to do). The shot with the dress in the surf doest appear in the movie but the dress does get soaked, in a very good rain scene at the 34 min mark. The whole movie is on youtube (only an average quality print)