As for maritime related things to do in the Detroit area, take a look at the Links page on the main Boatnerd site
http://links.boatnerd.com/. Look under a) Museums and Societies, b) Museum Ships and c) Tour Boats, Ferries, Charters & Cruises.
Besides the Dossin Great Lakes Museum on Belle Isle, I'd suggest the museum ships Willis B. Boyer in Toledo, as well as the lightship Huron and the buoy tender Bramble in Port Huron. And take a cruise on one of the Diamond Jack tour boats. They offer several longer cruises each year for boat enthusiasts as well as their regular 2 hour public tours. The next on is on September 25th
http://www2.boatnerd.com/media/WhistleBlowFlyer10.pdf
I always enjoy a ferry ride, so whenever I travel to Ontario, I try to take either the ferry at Algonac or the one at Marine City. Nice relaxing, if short ride, and the line at customs is never more that a boatload of vehicles long. There's also the ferry to Harson's Island near Algonac that doesn't cross the border.
If you just want to sit back and watch the ships pass in a park like setting, there is Belle Isle in Detroit, as well as numerous parks along the St. Clair River in both the US and Canada. Stop by the Boatnerd World Headquarters in Port Huron
http://www.boatnerd.com/about/worldHQ/default.htmto get a cup of coffee and see what boats are in the area.
As for research, most of the large public libraries in Michigan have a good selection of books and reference material on Great Lakes shipping. The Toledo-Lucas County Library has a good selection as well, but the largest shipping related collection in the area is the Historical Collections of the Great Lakes housed in the archives at Bowling Green State University.
http://www.bgsu.edu/colleges/library/cac/page38714.html
Thirty dollars a year gets you membership in the Marine Historical Society of Detroit, which includes monthly newsletters as well as an annual calender plus numerous gatherings and presentations each year.
Finally the annual Know Your Ships book
http://www.knowyourships.com/ is almost worth its weight in gold for boat watchers, considering all the information it contains.
An addictive hobby? That's an understatement! I tried to cover the most popular family-friendly things within an hour or so drive of Detroit, but I'm sure I missed some. Expand that range outward and you could say busy every weekend and never see it all.