The exercise instructor at the pool (in the pilot) was Karen Voight
http://en.wikipedia.org/wiki/Karen_Voight
Her name is not not listed at IMDB and you can see hardly her face (all the more her body :) since she is filmed from the side respectively from behind, but Tom Mankiewicz mentioned her explicitly in his audio commentary on DVD.
And another fun fact mentioned in the commentary:
Just before the first victim drives off a cliff, he turns on the radio and you can hear "You Must Have Been a Beautiful Baby" in a version sung by Stefanie Powers herself!