The Blue Water side of the state is a river story before it is a lake story. The St. Clair carries freighters past Port Huron, and the towns along it sit close enough to the bank that a person can walk from a downtown street to the water without a resort between them.
Port Huron is the clear start. The river leaves Lake Huron there, Fort Gratiot Light marks the outlet, and the Blue Water Bridge crosses upriver to Sarnia. South of the city the river continues as a working channel, with small towns on the Michigan bank and Ontario on the other side. The lake people mean, when they say the lake, is behind you once you drive south. St. Clair and the river towns below it are still on the river. Lake St. Clair is farther down. It is a different water, and it is not required for this day.
Go early. The useful hour is before the day fills in, when the channel is the fact in front of you and the main street is still a local street. Cross the bridge only if Canada is the plan. Otherwise stay on the Michigan bank and follow the river path until you have actually been beside the water. A view from the car at the bridge is not the same walk.
Early October is a plain month for this. The freighter season does not depend on foliage, and this piece does not promise a ship in a particular hour. What it promises is the order: river town first, open lake only when the river has actually delivered you there. Reversing that order makes every town a blur on the way to a beach.
The waterfront along this stretch is public land, a working channel, and a weekend destination at the same time. On a Sunday those uses share a bank. On a Tuesday the weekend use drops away and the channel remains. Either day, the towns are river towns. The lake is what the river is headed toward, not a substitute for them.