Lisa Gerritsen, who plays Cloris Leachman's worldly young daughter, Bess, on the new sitcom Phyllis, has trod the boards for seven years and the water for three. In a TV season distinguished principally by disaster, Lisa and Cloris and Phyllis have managed to stay afloat. Alas, Lisa on water skis cannot say the same.

The reason may be that the 17-year-old Lisa, who will enter UCLA at San Diego next March as a freshman, disdains bipedal technique. "Two skis are just for learning to get your balance," she says, "Everyone uses one ski."

Is it scary? Not at all. "When I think I am going to fall," she explains cheerfully, "I just let go."

At Pyramid Lake, Calif. recently, Lisa demonstrated her what-the-heck modus operandi, and with considerable splash. Then, after an all-points wipe-out, she ad-libbed a whale salute.

Luckily, unlike TV, taking a fall on water skis isn't particularly rough, even on the ego. "It's always a soft landing," says Lisa. "I think about that a lot." Needless to say, she'll study psychology at college.