We'd been enjoying big seafood dinners at Jonathan's Lakeside Restaurant (5622 Belle Grove Road, Linthicum,  789-2550) when suddenly, at nine, the lights went outout, not downand a band began to play. As couples boogied on the floor, under the revolving colored lights, we finished our meal by feel. Clearly, this is where Linthicum goes to enjoy a good time on a Saturday night. If you want quiet, go early, and hook some fresh fish, barbecued ribs, and some of the tastiest steamed shrimp we've eaten. The hot crab dip is a stunner, and fills a whole bread bowl to overflowing.