The earth is seven-tenths water, and so are we; the tides of our mutual attractions are never stronger than in summer.

See now, at the Bethesda pool, the seven ages of man--the waders and waddlers, the piggybackers, the belly-floppers, the show-offs, the sunbathers, the supervisors and the lappers.

Tiniest are the shallow waders, speechless and solemn in their own pool. The preschoolers, kicking and spluttering, cling to the crook of their parents' arms. The poolside divers, aged 7 or 8, pause with their arms pointed prayerfully overhead before sacrificing, stomach first, to the stubborn water.

The splashers and high-board divers, 15 and 16, are a freakish combination of bravado and belligerence, begging for attention and then fighting it off. The bathing beauties, sleek with oil and bright with Lycra, lie on their hips with their knees casually crooked for the slimmest effect.

Behind them sit the guardians and the car-pool drivers, the designated mothers, tilted to the sun with a casual caution; and beyond them in the water, the elders of this miniature community, patiently breast-stroking under pallid rubber caps.

In this suburban Shangri-La on Little Falls Parkway, between the faint laundry-room smell of chlorinated water and the coconut seduction of tanning oil, whole days and weeks and summers drift by.

There is a peculiar defense of innocence here. High-chinned and careless, these bare sojourners arrive two by two in the security of sexual separatism. The young men flourish cigarettes and Sony Walkmen, the young women barricade themselves behind Glamour magazines. In the reflected glares of hot sun and bright water, they come to be seen, not herded.

"It's a scene, but it's not the scene," says a consciously svelte young woman with fuschia nails and magenta swimsuit, lined up early to secure a good spot in the sun. "There's too many tubs around, and not enough talent."

Talent, as she views it, is only skin deep; that is the law of pool life. Beauty, poise, wit are long-distance virtues, and a good tan covers a multitude of sins.

"Every time I move, I feel like I'm coming out of this suit," says one of four girls whose slingback chairs are regimented toward the sun.

Their distance from the sun-worshiping women drives would-be suitors to drenching displays. Mike Croce, 16, wants the widest waves from his cannonballs, the hardest spray from his splash battles. He and Richard Boehme, 21, slap the water like warring beavers, drawing squeals of mock terror from their younger admirers.

They also draw the Morse code remonstrances of the lifeguard's whistle, a perpetual piping that blends into the general din: the Top 40 whine of big box radios, while over the pool hangs a continuous chord--the shrill watch-me squeals from the shallows, the tenor threats of underwater taggers, and the deep Geronimo grunts of Croce as he lunges off the high board.

Croce and Boehme are the Laurel and Hardy of the Bethesda pool. Croce, blond, blunt and a little gangly, has the attitude; Boehme, broad-shouldered with a gold chain like a lei on his island-like tan, has the car.

They met here, in a splash duel, two summers ago, and now they spend a couple of hours each afternoon at the pool, basking in a narrow notoriety. Younger boys imitate Croce's almost clumsy swaggering; younger girls creep up to Boehme's blanket to steal his car keys.

Neither has a job. Boehme, who used to have a job ministering to the members of the Woodmont Country Club, mows lawns for gas money to cruise his '72 Impala. Croce, who quit a sporadic job at his uncle's restaurant, does odd jobs and "bums off my mother" for pocket change.

By midafternoon they'll be gone, first to pick up Boehme's girlfriend and drop her at work, then maybe they'll drive up to Wheaton to kill the afternoon with Croce's brother. Maybe a movie, then tomorrow back to the pool, kicking around the gate before opening time.

"No talent today," sighs the magenta maillot, wrapping herself in a towel sarong. "Gotta go . . . see ya tomorrow."