Oh ya had to go and get me started...
They close the beaches due to high e.coli counts. e.coli is a fecal coliform bacteria (it lives in fecal matter) that in and of itself isn't really a big problem but is an indicator of other types of intestional critters that CAN cause some problems.
Here's the thing with the beach closings, they collect the water samples early in the morning. The test is a 24 hour culture test. If the culture shows high bacteria, they close the beach. So, they'r closing the beach on the day AFTER the bacteria was collected, you see what I'm saying? They never know what the ACTUAL bacteria counts are until the day after they close the beach.
And that is only certain beaches that have that problem. The e.coli comes from the poop of birds and stuff. So, its highest at Washington Park and some of the beaches in the state park.
I understand you being concerned about your first pup! But, don't worry about Lake Michigan. She'll be fine swimming in the lake. And so will you! It doesn't get warm until like, the end of July or August, though! So, it might be fun for Madi but not you!!!