I have to correct
wampus's writeup, to say Lake Michigan is full of nastyass chemicals is like saying that because the beaches in
Boston Harbor are sometimes closed, and because their are
PCBs in the
Hudson River where it empties into
New York Harbor, that therefore the entire US Coast of the
Atlantic Ocean is unswimmable. The
Great Lakes are the largest fresh water deposit
in the world.
The vast, vast majority of Great Lakes shoreline is fine for swimming.
Heavy metal and similar contaminants in fish harvested from Lake Michigan are much lower than just 20 years ago, to the point where its only a concern if you eat something like one fish a week, all year long.
In fact, if you camp on one of the Lake Michigan islands, like Garden Island, you can drink water straight from the lake, although you should boil or filter it to get rid of naturally occurring flora and fauna.
But, wampus' testes are not completely out of danger -- it is indeed damn cold.
My source for this opinion is my lifetime of personal experience, and also http://www.epa.gov/glnpo/monitoring/swimmability.html. (and my boys are fine, thanks for asking)