Lost At Sea (Francine Pascal's Sweet Valley) by Kate Williams. bantam paperbacks 1989.
Shipwrecked ... It's a beautiful, sunny day when a group from Sweet Valley High sets sail for a special science field trip. Jessica Wakefield is looking forward to a few hours of sunbathing and flirting, while her twin sister, Elizabeth, can't wait to get to deserted Anacapa Island to study the marine life. Neither of them could have imagined the nightmare the trip will become. On the way back from the island, a violent storm sets in, and the group is forced to abandon ship in the middle of a raging sea. Then, when the lifeboat carrying Jessica and Winston Egbert capsizes, they disappear into the fog. After the storm clears, Jessica's life preserver turns up. And soon the Coast Guard spots the missing lifeboat--but it's empty! Will Jessica and Winston ever be found?11