I didn't grow up in Boston but spent a lot of time sailing (and accordingly, a decent amount swimming) in the Charles River Basin in the past several years (go

I didn't grow up in Boston but spent a lot of time sailing (and accordingly, a decent amount swimming) in the Charles River Basin in the past several years (go Tech Sailing!). Only once ever had a bad experience - got terribly sick for 24 hrs after a windy windsurfing day where I was in the water more than I was on the board. I was told this was "river fever" and it happens from time to time! Otherwise a great body of water. The days the Charles is unswimmable is a few hot summer days after rainstorms (and in the winters when it freezes up!). In the summer the water is warm enough and the runoff after storms brings in enough nutrients from the city that cyanobacteria grow like crazy and create a paralysis risk. On these days the Boston DCR does a pretty good job making sure everyone knows to stay dry for the day.