That secret little seafood restaurant, with terrific fresh fish at bargain prices, that one that you dreamed about in your restaurant fantasies, has come true, and it's called Captain Days. It is in an alley between 18th and 19th streets, south of M Street, and it really is a bargain, with lobster under $10, blackened redfish at $7.95, and mesquite-grilled red snapper at $7.95. It is a casual place, for eating as well as drinking, and the waiters and bartender are friendly sorts, which sometimes counts more than efficiency if you're not in a hurry.

You can choose from specials on the blackboard, but the stuff on the menu is just as good: soups (clam chowder and seafood gumbo being particularly good), raw bar standards and barbecued shrimp, seafood salads, sandwiches and entrees that include fried seafoods,steamed lobsters, baked stuffed flounder and a half-dozen kinds of fish grilled over mesquite. The only meats on the menu are chicken salad and barbecued chicken. Fresh vegetables were another nice surprise in such a casual restaurant.

Captain Days' location suggests that lunch is its prime time, and its prices encourage dinnertime luxury at midday. 1130 Rear 18th St. NW. 296- 1133. L, D $4.95-$12.95. L, D daily. Closed Sun. AE, MC, V. Full bar.