The Duncan Bay is where Cheboygan State Park is. In the bay there are two shipwrecks that were just a blast to kayak over. The bay is accessible through the state park or there are several points you could access in Cheboygan. The bay mouth can get big waves but if you start from the state park, the bay is fairly protected.
There are a lot of recreational boats in and out of the Cheboygan River/Marina, though they do not spend much time in Duncan Bay. The one shipwreck, "The Leviathan", sits at the mouth of Duncan Bay on the south west side of the bay. It was great. We had lunch there on the beach - part of the state park.
There are actually two wrecks right there - the other is the "Genesee Chief" - but not much is left of that one. It was still a blast to check out.
Duncan Bay on the south side is all State Park.
MI State Park Sticker
From Cheboygan head south on 23 to the State Park. Or if you want a greater challenge, then start from the city park in Cheboygan or from the marina.
Shipwrecks of the Straits of Mackinaw is one spot that holds information about the wrecks.