I used to live between Mackinaw City and Cheboygan when I worked at the USFWS' Sea Lamprey Research Laboratory on Hammond Bay, now under USGS oversight. Yes, I thought the same thing when I first saw French Farm Lake. While it does get birds on it, particularly when there is a blow in the Straits' open waters. It doesn't build-up birds, they just don't linger. Carp Lake (changed to Paradise Lake by the local Chamber Officials) is more productive. Brown's Marsh west of the toll booth complex (gravel ramps) can hold good numbers of birds, as do the coastal marshes east of St. Ignace ( Under the Consent Decree there is a Tribal fishery for Soo and Brimely Band members on salmon here-watch for poorly marked Tribal gillnets set off Nunn's Creek). The are two other good waterfowl lakes to the south, but they are the hunting sites of a former co-worker and still current friend.
French Farm Lake is an old embayment of Lake Michigan that was cut-off as the landmass rebounded from ice sheet compression following the last Pleistocene continental glacier retreat. It is a bog dystrophic lake, so inherent productivity is very low. The surrounding watershed is not very productive either-sand soils dominate.
I have taken mosly ringnecks in the north end, with some late goldeneyes. A fair number of swans use it as a roost lake as well when they migrate through norther lower Michigan. If your trip coincides with a good storm and northwest blow, you may do quite well.
The marsh complex off the south end of Mullet Lake can be very good hunting, just north-east of Indian River. If you are a diver hunter, Crooked Lake can be very productive, just be prepared to be challenged by some irate shoreline $400,000.00 home owner for disturbing their ambiance.
My best,as well as most consistent, hunting in this area was opposite my in-laws old home on the Spreads section of the Cheboygan River upstream of its confluence with the Black River-puddle ducks inshore and divers along the channel if you set-up in the stump fields. I would assume development along the shoreline has brought about the end of this hunting spot, since it was seven dogs ago.
