All of Marin County’s beaches received high marks for water quality on dry summer days last year. Heal the Bay, a Santa Monica-based environmental nonprofit that grades California beaches according to the likelihood of falling ill from contaminated water, gave A grades to all 24 beaches tested in the county. The grades consider levels of coliform, E. coli, and Enterococcus bacteria, as reported by the county. “The beaches we monitor are doing exceptionally well,” said…