BOAT-RENTAL AGENCIES are prevalent at Southern California lakes, and all kinds of boats can be rented. At Big Bear Lake, for instance, the marinas rent out canoes, kayaks, rowboats, wind surfers and pontoon boats. Jet skis are also available. Isabella Lake, in Kern County, has water-ski boats for rent; and paddle boats, canoes, aquacycles and now sailboats are available at the lower lake at Castaic Lake. Lake Elsinore's selection includes rowboats, pontoons, pedal boats, surfboards and floats. Lake San Antonio, 30 miles from Paso Robles, is one of the few lakes featuring houseboat rentals. Charter and excursion boats are available at Lake Arrowhead.