Open for lunch daily 11:30 a.m. to 2:30 p.m.; for dinner 5:30 to 10 p.m. AE, MC, V. Reservations are taken until noon for lunch and until 6:30 for dinner. Prices: for lunch appetizers $2.95 to $3.95, entrees $5.95 to $8.95; for dinner appetizers $2.50 to $4.95, entrees $8.95 to $14.95. Full dinner with wine, tax and tip about $30 to $40 a person.

One of the mysteries that remains after decades of studying restaurants is how one place can specialize in a very

narrow range of food or cooking styles -- and do it badly.

It is far easier to understand and to forgive a restaurant

that tries many things and succeeds at only some of them than one that tries very little and fails at that.

Take Potowmack Landing as an example.

It is a seafood restaurant, although it does offer a couple of grilled steaks and chicken. And it cooks primarily by mesquite-grilling and deep-frying.

Although it is a seafood restaurant, its repertoire is narrow within that genre. No lobster; only one crab dish, crab cakes; and only four fish -- halibut, grouper, flounder and swordfish -- offered regularly, with perhaps one such as trout for a special. Shrimp are deep-fried, saut,eed with garlic and parmesan, spiced or grilled en brochette. Scallops are deep-fried or broiled on a combination platter. Clams are raw, topped with bacon and green pepper as Clams Casino or in a chowder. Seafoods are combined as a broiled or fried platter or in a tomato sauce as cioppino. This is as pedestrian a seafood menu as one might devise with the exception of the mesquite grilling and the swordfish served raw as sashimi (a mistake, as far as I am concerned); it is a seafood house that makes tomato curry thesoup of the day and chicken aloha rolled in coconut a daily entree.

What's more, the worst of the food at Potowmack Landing is likely to be the seafood.

What has been good? The breads are a basket of yeasty and crusty seeded sticks, nutted muffins and drop biscuits, which are not wonderful but a pleasant offering. The vegetables are also more worthwhile than most, whether tiny saut,eed mushrooms as an appetizer or saut,eed squash and carrots as an accompaniment. The baked potatoes are fine, accompanied by sour cream, chives and real bacon, cooked without a foil wrapper. And the cauliflower that came with one day's dinner was fresh and garlicky, certainly delicious.

The wine list is short and California accented, and there are some good choices, though the '79 Firestone Merlot I had at lunch elsewhere for $9.50 was $15 here. Furthermore, the manager declined to take back a bottle of chardonnay we thought undrinkable because he considered it good. He should have suggested some concession after we ordered another equivalent bottle and left the first unfinished.

Now, to what really doesn't work at Potowmack Landing: the buying and cooking of seafood. The appetizers of raw oysters and clams are all right, though smaller and less plump than one might like. Spiced shrimp are seasoned brightly but the shrimp themselves have no flavor and some crumbliness in the texture. Clams Casino are hard to ruin, unless you use large, chewy clams, which Potowmack did. And the crab gumbo could use both more crab and more seasoning, while seafood chowder had more seafood but a tinny undertone to its creaminess. At least the appetizers were reasonably pleasant. Less praise can be given the main courses.

You can watch your fish being grilled through the glass walls of the kitchen. But what you can see is the unfortunate grill cook being driven back by the smoke; something doesn't seem to work right in there. Perhaps it is flare-ups, for some of the fish has ha a sooty taste beyond the mesquite flavor. The grilling was reasonably accurate, although the grouper one night was on the bouncy side, but the fish was utterly devoid of taste. From the flounder to the swordfish to the grouper to the halibut, the lack of flavor was so pronounced that nobody was inclined even to half-finish a portion. Steak was better -- besides being well trimmed and cooked as ordered, it was agreeable to eat, though not particularly juicy or thick. Fried foods held an appalling amount of grease in their thick, heavy batter, which was gummy inside and overwhelmed the shrimp or zucchini or whatever was meant to be featured. And cioppino was an impressively large pot of tomato-based seafood stew, but the sauce was sweet rather than spicy, and the seafood in it ranged from juicy mussels and chunks of fish to adequate shrimp to raw scallops beyond freshness and other fish chunks that were bitter or chewy. The single main dish with any claim to excellence was crab cakes, which were mildly but definitely seasoned, light and plump and substantial with good crab. Accompaniment choices besides the vegetables and baked potatoes were a dry, dull bulgur pilaf, low-grade limp french fries and coleslaw that tasted like oiled cabbage with a splash of vinegar and seeds.

Which means you would probably want dessert to fill you up. Those choices include an "apple pie" that is more like an apple-flavored nut cake, and not bad if you are braced for that. The lemon mousse is fluffy and passable; the ice cream is Haagen-Dazs, they say, and generously served in a liqueur parfait. There are supersweet fruit and nut cake, bitterly grainy chocolate terrine, and a fruit and cheese plate that would make a nice little lunch, since it is kiwi, strawberries and oranges -- hardly the accompaniments to cheese but nice for a fruit salad -- with a couple of respectable cheddars.

Lest you think the food is the only problem, consider the acoustics in this cheery high-tech dining room with nautical flags. The high, exposed-beam ceiling absorbs conversation before it can travel across the table.

So why consider Potowmack at all? Simple: It has a glorious and sweeping view of the Potomac from the Virginia side. If you can't eat on your boat, Potowmack may be your best alternative. The waiters are awfully nice, and you don't have to worry about your soup spilling when the wind shifts.