ASSAULT ON LAKE CASITAS By Brad Alan Lewis (Broad Street Books: $11.95, illustrated) . A gold-medal winner in doubles sculling in 1984, Brad Lewis offers a look at the fanatic dedication required of Olympic athletes. Initially cut from the rowing squad at the Olympic-selection camp, Lewis and his partner, Paul Enquist, practiced on their own and won a berth on the final team. His descriptions of their almost monomaniacal regime of physical and mental training is straightforward and honest: Lewis doesn't shrink from painting himself as an unsympathetic figure. Unfortunately, he fails to look beyond the narrowly circumscribed world of Olympic competition, as Stephen Kiesling did in "The Shell Game: Reflections on Rowing and the Pursuit of Excellence"; Lewis seems to regard President Carter's decision to boycott the 1980 Olympics as a personal affront.