River rats should be out in full force this Saturday for the 13th Annual Ramblin' Raft Race on the Potomac. It's the only day of the year that unregistered craft are allowed on the river. So leave those kayaks and canoes behind, throw together anything buoyant- looking hanging around the house and head on down. More than 600 homemade vessels, (the more creative the better), inner tubes (floating solo or strung together) and rubber rafts are expected to sign up. Some will be ready for a 1.7-mile race against the clock between Memorial Bridge and 14th Street Bridge, others will bob along leisurely and see where the current takes them. Spectators are invited to line the banks of the course, and there'll be live bands, mimes, jugglers and magicians providing sideline entertainment. A 600-foot submarine sandwich is being prepared for the occasion; pieces will go for $1 a slice. Registration is between 7 and 10:30 a.m. the day of the race, and costs between $1 and $10 depending on the size of your craft. The race gets under way at 11; and the starting line is in West Potomac Park on Ohio Drive SW near the Jefferson Memorial. Call 534-0320 for more information.