In the 1970's, swim wear is all about being sexy. Sheer suits are introduced as well as the thong. String bikinis also become very popular. The best photos I could find to showcase this come from Sports Illustrated. The SI Vault has a great selection of some of their best shots and models. Here's a few:
Monique Moura de Carvalho in a string bikini. This is a beautiful photo.
Christie Brinkley in a sweet and sexy one piece.
Christie Brinkley in a thong bikini.