Unlike the Charles, which gets a healthy B+ report card grade from the Environmental Protection Agency, the Mystic gets a D. That means that bacteria levels are unsafe for swimming 48 percent of the time. ’’It's a little unfair to compare the Charles and the Mystic, ‘‘’’ says Dave Deegan, New England region, EPA. ‘‘The EPA has intensely focused on the Charles for 11 years. This is the first year we are concentrating on the Mystic. We would be delighted to get to the point where it is as equally clean.’’