Captain John Smith found the Potomac River sparkling clean and full of fish when he sailed upriver to what is now Washington at the opening of the 17th century; he went home raving about it.

Captain Jacques Cousteau, who will be arriving in Calypso Saturday morning, will not find the river running anything like so clear and rich near the close of the 20th century, but it could be worse.

Hey, it has been worse, lots worse, and it's getting better all the time, you should go see and enjoy. That's the message of this weekend's Riverfest, the second annual celebration that Mayor Marion Barry Jr. has laid on to remind us of our waterfront, our water source and our principal scenic setting.

There will be no end of things to see and do along Water Street both Saturday and Sunday, but first, to get a sense of the river, beg, borrow or rent a canoe and slip along the river in the early dawn. Keep down the chatter and feather the paddle and you'll very likely see herons, ospreys and eagles.

Anywhere upriver from Roosevelt Island you may be startled by the tailslap of a beaver you've startled, or find yourself under examination by a bright-eyed otter. Deer come to drink along either bank.

But as the sun rises so does the city, and this weekend that'll mean it's time to go racing or go ashore. Sailing regattas and a canoe paddle-thon will rule the river, once Calypso and her escort of tall ships have passed.

The exercise will have whetted your appetite, and there will be scores of vendors around the harbor to satisfy it. On the promenade (harbor side) of Restaurant Row the big sit-down eateries will be dealing stand-up nosh, mostly seafood; along Water Street there'll be vendors of everything from hotdogs, popcorn and peanuts to West Indian curried goat, fried wonton, Moroccan covs, fried bananas, shiskebab, tacos, enchiladas and beef Stroganoff. Wash it all down with soda or beer; top it all off with cotton candy, ice cream or Italian ices, Chicago-style (now, that's Italian.)

Perhaps the grandest scene short of the festival-ending fireworks will be the spectacle of the tall ships in narrow Washington Channel as they come to dock after parading upriver with Calypso. Time was when the Maine Avenue docks were always a forest of masts, as you'll be able to see from the early photographs in the river heritage exhibition tent on the promenade near the Barley Mow Restaurant.

Some things you'll want to know:

* Timing is everything. The festival runs from 9 a.m. to about 10 p.m. Saturday and resumes at 11 a.m. Sunday. It ends with a whole bunch of bangs beginning at 10 Sunday night, when the Zambelli family lights up both river and River City with a half- hour fireworks display.

* Metro is the way to go. Parking is nowhere near adequate for such crowds as Riverfest turns out, and the L'Enfant Plaza Metro station at Ninth and D streets SW is only a few blocks from the waterfront, less than a 10-minute walk. There'll also be free shuttle buses to Seventh and Maine from 9 a.m. to midnight both days. The Sunday Metro operating hours will be extended to midnight, giving you plenty of time to catch a train after the fireworks extravaganza ends at 10:30.

* The Anacostia's our river, too. There will be free shuttle buses to the Washington Navy Yard from Seventh and Maine from 11 a.m. to 5 p.m. both days, which gives you a chance to visit Washington's most-overlooked major museum. The U.S. Naval Museum has periscopes you can peer through, antiaircraft guns you can crank through their paces and a whole vast cavern full of other neat stuff. Out at the dock is the U.S.S. Barry, waiting to welcome you aboard.

* If you lose your mother, go to Seventh and Water streets. If somebody hasn't turned her in already, they will holler for her over the PA system. There will also be a lost-and-found there for less animate objects.

* The rest rooms are on the Water Street side, at Ninth, Seventh and Sixth streets.

* Most water demonstrations will be in Washington Channel, as near the promenade as possible, in the area opposite where the former presidential yacht Sequoia is moored. Exceptions are noted in the schedule.

* Leave your dog at home.

* Don't go in the water. While the Potomac is far from being the pestilent conduit it was within the past decade, it's not yet safe for swimming "or other extended contact," as the Coast Guard puts it.

This reminds us that, while cooperation among national, state and local governments has accomplished a lot, there's lots still to be done. If you want to know more about that, you might sign up for Wednesday's lecture cruise of the Washington tour boat Spirit of '76.

Sponsored by the American Planning Association, the $12 cruise will include a discussion of what has been, is being and must be done to restore and preserve the Potomac, and an on-board reception. For information, call 587-6619 or 985-7195.