"offshore in Michigan" is different from
"offshore in Florida".
You'll get passable coverage on the lakes in Michigan, except for at the center of the lakes in rougher water conditions. In those cases, a cheap external antenna will help a great deal.
Offshore in Florida is "one sided" and you have reasonable ability to use your phone up to around 4 miles out. A big long (6' or 8') marine Cell antenna will extend that to around 12 miles (although I've gotten a good signal at 15 miles with an antenna tip height of 23').