I just did some on-line research and made some phone calls (I needed to get these into my cell phone, anyway):
If landing in the U.S. on the shores of the Detroit River or Lake St. Clair up to and including Harrison Township: (313)393-3793
If landing in the U.S. on the shores of the St. Clair River or Lake St. Clair down to and including Harrison Township: (810)985-9972
As you can see: If you're landing in Harrison Twp. you can flip a coin.
If everybody on the boat doesn't have either a NEXUS or I68, you will likely be told to proceed to a specific check-in location. If everybody on the boat does have either a NEXUS card or I68, they will probably tell you to proceed to wherever you planned to land. They may or may not indicate they wish to inspect boat and passengers in the latter case. If they do wish to inspect: They'll usually agree to meet you wherever you intended to land, but no passengers or goods may leave the boat until DHS CBP arrives.
If you, as boat owners, have NEXUS cards instead of I68s, you can still ask for a Boat(er) Registration Number (aka: "BR Number"). Then DHS CBP can relate your NEXUS information to your boat's information (type, colours, size, name, HIN, etc.), and simply ask for your BR Number on your return, which saves a lot of Q&A on each return.
Jim