(FORTUNE Magazine) – Sea kayaking, a sport that until recently was dominated by rugged outdoorsmen, is catching on with the less intrepid -- and their young children. Ken Horwitz, 38, a stockbroker with Paine Webber in Seattle, kayaks in Puget Sound with his wife, Brigitte, and daughters Katja, 4, and Emily, 3 months. Babies in kayaks? Sure. Unlike the white-water kayaks in which the truly adventurous navigate raging rivers, sea kayaks can be paddled through ocean bays and inlets. Horwitz says the gentle motion of the boat lulls Emily to sleep in his wife's lap. But, adds Horwitz, ''once we see whitecaps, we go back to shore.'' In 1990 an estimated 15,000 wood, plastic, aluminum, or fiberglass sea . kayaks were sold, up from about 11,000 in 1989, according to a trade association. The craft cost around $1,500 apiece, and some models can be folded to the size of a golf bag for easy toting. Retailer L.L. Bean runs kayaking courses in Freeport, Maine, that cost $15 a session. Horwitz, who owns 15 sea kayaks, teaches the sport too -- free, as a Red Cross volunteer. - J.R.