IT HAD BEEN a perfect canoe outing on Rock Creek until Robert Sands "hit this area that was all airy and foamy . . . The canoe just filled up and sank right out from under me." While he floated off in his life jacket, the canoe bounced along the bottom until some rocks rose up to embrace it.

By the time Sands could retrieve it, the pads on the carrying yoke had been ripped away, the plastic hull was torn, both aluminum gunwales were badly bent and a seat was broken.

For about $200, he got the canoe fixed by David Brown of Great Falls Canoe Repair. For free, he got two words of advice on how to prevent so much damage: Add flotation.

If Sands had, the canoe wouldn't have sunk or been wedged where the current could tear it, says Brown, who has reconstructed more than 2,000 torn, twisted, broken canoes in the last decade.

"No matter what kind of canoe you have," Brown says, "if you are going to paddle on creeks and rivers, add extra flotation. If the canoe is plastic, put in extra flotation even if you stick to lakes, because those boats don't have much inherent flotation."

For flotation you can use airbags, foam blocks, waterbeds filled with air or innertubes. Any of them will displace water and help the boat float high enough to prevent major damage even after a capsize. But, Brown cautions, "make sure you secure it to the boat so it doesn't float up and away by itself just when you need it."

Brown also recommends:

* When you carry a boat on your car, tie it down four ways, front and back and twice across the hull.

* Store the boat securely, that is, tie it down if it's in the open where the wind can blow it into a tree or wall, or keep it in a sheltered place.

* Customize the boat to fit you. That's especially important in whitewater, Brown says, where you want the canoe to function around your body. "You can't really control the boat if you are slipping and sliding around inside of it."

To make the boat fit you, you may need to lower the seat or paddling thwart and adjust its location to account for your weight.

* If you use knee pads, put non-slip abrasive strips, like those used in bathtubs, on the bottom of the hull where you kneel. Or skip the knee pads and glue in squares of wetsuit- type neoprene rubber to kneel on.

* Learn to paddle properly. "It is really important that you know what you are doing if you want to paddle rivers and creeks," he says. "Even if you have canoed on lakes for years, take whitewater lessons. I have done both, and I know it is a quantum leap from one to the other."

The difference in difficulty between a fast, demanding stream and a lake, "even on a windy day, is beyond your wildest imagination."

* Store all loose ropes under shock cord or rubber strips riveted to the deck plate.

* In plastic canoes, add extra thwarts.

For help in getting your canoe customized -- or restored, if Brown's advice has come too late -- you can reach him at 281-4380.