Here in Michigan, all inland lakes (not lake Michigan) and rivers have a 55 mph speed limit unless posted otherwise (always less than 55, never more). Some have time limits.....no wake from 7:00 p.m. to 10:00 a.m. and things like that. And Lake Michigan DOES have a speed limit. If you are within 1 mile of shore the speed limit is 55 mph. I've never seen it enforced, and don't think they ever could. As was stated above, radar doesn't work real well on the water (fiberglass doesn't give a good signal) and determining if you were within one mile could be really hard.
I do know that the 26 American Offshore that Tinkerboater and I got a ride in at Hardy Dam a few weeks ago got a speeding ticket for going 102 the next day!!! And knowing Fred, I'm sure he is REALLY pissed and hot under the collar!!!!