I finally found the movie I was looking for!
It’s called After Office Hours (1935) with Constance Bennett and Clark Gable.
http://www.imdb.com/title/tt0026043/?ref_=kw_li_tt
I was mistaken in thinking that the party and boathouse scene are at the start of the movie....they’re about halfway through. The only place I can find to watch online is here...
https://m.ok.ru/video/276415711907
but I’m ordering a copy from Amazon.
Thanks for your original reply and sorry for the mislead on the sequence of events.