[Matt]

Beaches are closed primarily due to contaminants (fecal coliforms or other microbial contaminants) which is usually tested for daily, weekly, or monthly depending on budgets.
And the number of closings per year are how beaches are rated by the Natural Resources Defense Counsel
How do our local beaches rate?

Not too well... monitoring samples exceeded the state’s daily maximum bacterial standards in some of our most popular beaches.

Niagara County had the highest percent exceedance rate in 2008
(29%), followed by Wayne (26%), Monroe (24%), Erie (22%),
Chautauqua (21%), Westchester (8%), Cayuga (7%), Suffolk (7%), Queens (6%), Nassau (5%), Richmond (4%),
Jefferson (3%), Kings (2%), Bronx (1%), and Oswego (0%).

Here is the full report with beach specifics
http://www.nrdc.org/water/oceans/ttw/sumnewy.pdf