Denis Crean swims in the Potomac River two times a week. He takes others in, too, to teach them techniques for traversing the open water.

The most common response when he tells people, he said, is shock: “You go swimming where?!”

But those who monitor and patrol the river said getting people back into the water has always been the goal. To that end, the Potomac Riverkeeper Network announced Tuesday it will monitor water quality from six points along the river using a floating lab built into a 42-foot boat called the Sea Dog.

The data will be publicly released and uploaded to an app that allows people to check the quality of waterways and decide whether the water is safe enough for a swim.

Swim Guide, a website and app that tracks weather and water quality for about 7,000 beaches around the world, relies on data from water samples to issue color-coded ratings for waterways.

Green means good. Red means bad.

On Tuesday, when the riverkeepers christened their newly donated boat, the Potomac River was rated red for no-go — based on a September 2018 water sample.

In the District, the Department of Health bars swimming in the river. It’s not outlawed in Virginia or Maryland. But, as Potomac riverkeeper Dean Naujoks said, no one knows what the river quality is day-to-day or even week-to-week.

“They do a weekly swim out of here, and we start getting calls from people, like ‘How safe is the river out here?’ And the truth is, there is no data. There’s no bacteria monitoring here. We just don’t know,” Naujoks said. “There was very little data in D.C., there was almost no data south of D.C., and we just felt it was really important that the public know when they’re getting on tubes and water skis and paddle boards and falling in.”

That’s where the Sea Dog comes in.

Using a mobile laboratory given to the organization by an anonymous donor, the riverkeepers and a band of volunteers will test the water for a variety things, including temperature, pH level and turbidity.

But the most important metric, Naujoks said, will be bacteria readings.

Although the organization will begin monitoring water quality at six sites — two in Virginia, one in Maryland and three in the District — the riverkeepers hope that number will grow over time.

“This allows us to do this stuff right on the water. It’ll save a lot of money and make the whole process more efficient,” Naujoks said. “This creates a great opportunity for people who care about this river to go out and do water-quality monitoring. This boat will allow us to do it right.”

Last year, the Potomac River scored its first “B” on an annual river report card issued by the Potomac Conservancy, thanks to declining pollution levels, and the return of bald eagles, waterfowl and other native species. The organization also noted the expansion of protected forests up and down a watershed that stretches across more than 14,000 square miles.

It was the highest grade the river had received in a decade, up from a B-minus in 2017 and a D in 2011.

On Tuesday, riverkeepers and environmental advocates gathered along the dock at National Harbor as Nancy Stoner, president of the Potomac Riverkeeper Network, welcomed the Sea Dog with a toast and second christening as the boat entered a new phase of its journey.

“Today, in the name of all who have sailed aboard this vessel in the past, and all who may sail aboard her in the future, we ask that the gods of wind and of the sea to favor us with a blessing today,” Stoner said.

As she spoke, retired Navy Adm. J. Paul Reason looked on, smiling.

Reason, a four-star admiral who served for more than 30 years, including as the commander of the Atlantic Fleet, designed the boat more than a decade ago.

Based on a traditional Chesapeake Bay deadrise, the Sea Dog was made to be a family boat: A vessel that took Reason fishing and shuttled him and his wife, Dianne, up and down the Chesapeake Bay.

On Tuesday, the blue nautical-themed cushions and throw pillows that accompanied them on their journeys still lined the cabin. A day bed sat where the lab would be installed.

Reason, 77, said he wanted to give his boat to an organization that would make use of it.

“We found really no better utility for this boat that was really like a member of our family than to contribute it to the riverkeepers, because after all the years at sea and going into many rivers of the world, I have gained a significant regard for those who are titled riverkeeper,” Reason said. “It means a lot to us. I grew up on this river.”

Little by little, with the help of the boat, the lab and regular water readings, advocates hope to turn the Potomac’s image around.

Instead of hearing shock and disgust when he tells people he goes swimming in the Potomac, Crean said, he looks forward to the day when no one will have to think twice about getting in the water.