WANT TO GO to the beach? Why? Want to watch your motor fry within bleeding distance of the Bay Bridge, subject your children to the bone-chilling, leg-crushing waters of the Atlantic, share the tides with a crowd similar in dimension to the Last Judgement, and get sand up to here, in your ear and out your . . . .

Oh, never mind. Would you like to go swimming? How about a lazy lake or a peaceful river? Something with tree-lined shores, grassy knolls and butterflies; something your children can wade in safely; something your baby can sit in; something crying out for an inner tube and you in a swimsuit.

Pack a picnic, load up the kids and the car songbook and travel for an hour or so to some of the area's least known, least used and most wonderful summer spots.

Before we tell you what they are, we'd like to say a word about rivers and swimming holes. The word is, Unsafe. Spokesmen from visitors' centers and parks warned us repeatedly about the Rappahannock, Shenandoah and upper Potomac rivers, which they say look "deceptively easy" and contain hidden whirlpools, undertows and things on which you can stub your toe and worse.

But if you're reasonably cautious, there are a number of safe spots for swimming. Here's what we had in mind:

MARYLAND

BETTERTON BEACH --

Kent County, 301/778-1948. The 19th-century resort, across from Baltimore's harbor, has been spruced up recently. Swimming is in the fresh waters of the Sassafras where it meets the Bay; its salt-free waters, they say, discourage the nettles that so plague other Bay beaches. Amenities include a boardwalk, bathhouse, picnic pavilion, fishing jetty, pier and boat landing. Lifeguards are on duty 10 to 6. Best news: It's free. Weekends crowded "sometimes," they say.

To get there: Follow U.S. 50 (John Hanson Highway) over the Bay Bridge, head north onU.S. 301. Go left at Route 213 (at a light) into Centreville and cross the Chester River Bridge, said to be an experience in itself. Go five miles past Chestertown, and make a left onto Route 561. Go through Lynch, and turn right at the stop sign onto Route 298. Go a couple of miles, and make a left onto Route 292. Go through Still Pond and make a left at the stop sign (you're still on 292); follow directly to the beach.

CUNNINGHAM FALLS STATE PARK --

Thurmont. 301/271-7574. Here's where to take your school-aged children. The park has a gorgeous lake with all the amenities, including canoes to rent (from 10 to 5 at $4.20 per hour); you can hang out on the beach all day, or leave your teen-agers here to nosh at the food concession and flirt with the lifeguards. But take the rest of the family to visit the falls, a 220-foot adventure sure to bring out the billygoat in all of you. The falls are at a trickle now, but there's enough cold water to cool you off after the hot climb. If you can't make it during the week, try for Saturday; Sundays, they're overcrowded. Admission is $4 for Maryland cars, $5 for out-of-staters; half-price on Wednesday.

To get there: follow I-270 north toU.S. 15 (near Frederick); take 15 north to Thurmont. Exit onto Route 77 west, go four miles and turn left onto Catoctin Hollow Road. Go past the lake and turn right.

VIRGINIA

FAIRVIEW BEACH --

Near Falmouth. Everything we know about this comes from the regulars at Fairview Crab House across the river. They claim that Fairview is the first beach on the Potomac without nettles; it's also, they say, a safe place for kids ("you can wade out four or five hundred feet and still not be over your waist"). They charge $2.50 per person (kids under 10 are free), but offer no lifeguards. Saturdays are their busiest days; Sundays are better, they say. To get there: Take I-95 south to U.S. 17 through Falmouth; turn onto Route 218, and it will take you right to the beach.

LAKE ANNA STATE PARK --

Spotsylvania. 703/854-5503. This gets our vote for families with little ones. The waters are picture-postcard blue, gentle and shallow; the price is cheap; the picnic area's in the shade and the restrooms are clean. Near Fredericksburg, the place loads up on Sundays, so come on Saturday. We tried it in midweek and were almost alone. The park includes six miles of hiking trails (look for deer, fox, muskrat and beaver) and a boat ramp (bring your own boat), plus breezes from pine trees surrounding the 16,000-acre lake. Admission: $1.50 per car, $2 with a boat.

To get there: Take I-95 south past Fredericksburg to Route 606; turn west and go to Post Oak, where you turn onto Route 208 south. Go about 16 miles to the sign at 601; turn right (west) and go 3.3 miles to the park entrance.

WESTMORELAND STATE PARK --

Montross. 804/493-8821. Here's how to hook your children when you want to look at historic houses and they want to hang around the pool. The park's just up the road a piece from two historic birthplaces, those of Robert E. Lee (Stratford Hall) and George Washington. Amenities include a pool with lifeguards; river swimmers are on their own in the Potomac. There's a beach house, and you can rent both paddleboats ($3 per hour) or rowboats ($2 per hour). The park itself is open 6 to 10; admission is $1.50 per vehicle. There's a concession near the pool, but the picnic area, with grills, lies beneath enormous pine trees beside the river.

To get there: Take I-95 south to Fredericksburg; turn onto Route 3 east.

WEST VIRGINIA

CACAPON STATE PARK --

Berkeley Springs. 304/258-1022. A pretty two-hour drive from the District, the Cacapon attracts tourists to its CCC-built cabins and lodge. Daily visitors come for the six-acre lake, with its sandy beach and guarded shores (lifeguards on duty 10:30 to 6). Work up a sweat worth drowning by hiking up 2,300-foot Cacapon Mountain, looking for white-tailed deer and pileated woodpeckers. Or cruise the lake in one of their paddleboats ($2 per half-hour) or rowboats ($2.50 per hour). Fishing is allowed with a West Virginia license (get an out-of-state license at the lodge). Admission to the park for daily visitors is $2 per adult, $1.25 per child, free for kids under four.

To get there: Take I-270 north to I-70; go west to the second Hancock exit. Take U.S. 522 south through Berkeley Springs, and follow the signs.