I've been going to the beach several times a week for about 13 years, even during winter. Never seen it closed. They monitor water quality regularly and the results are readily available.

You're lucky. I live near the Baltic Sea, a relatively shallow sea with a very narrow link to the ocean, and agricultural runoff and sewage from 100 million people drain into it. We have very regular algae blooms, typically late summer when the water warms up sufficiently (for some reason, even though it was very hot, very little this summer).

The sewage problem is largely solved nowadays, thanks to modern sewage treatment plants, but thanks to the disastrous EU agricultural policy (CAP) there has been no improvement at all in the agricultural runoff for over 20 years.

Happens enough times in LA to have a page that tracks it: http://publichealth.lacounty.gov/phcommon/public/eh/water_qu...