Hey all,
This may be more of a hassle than not, but if that areas had been used as a dive mobilization site, you may have rights to use it. The legal aspect might also help you strike a deal for divers only, no other boaters or "riff-raff".
Depending on the length of time it has been used, the new landowners may be forced to let you still dive from that end. This would be known as an implied easement or as a form of adverse possession. Find a local surveyor and/or lawyer and they can fill you in on the specific requirements. However, it is very common for the courts to rule in your favor. Just making the owners aware of the possible outcome may help you reach an amicable agreement with them. They might be reasonalbe people once you talk to them and ask nicely....
For example, one person was given permission to keep cutting wood off another's property 'cause he'd been doing it for so long. Also, a landowner who used to let kids cross his property, but changed his mind after may years, had to remove a fence he had put up (kids father was a lawyer). These are not MI specific cases, and rules vary between provinces and states.