NOAA explains that true astronomical tides on the Great Lakes are very small compared with changes produced by wind and air pressure. Sustained wind can tilt the lake, and the resulting oscillation is called a seiche.
A seiche can affect clearance, dock lines, ramps and available depth. Conditions at opposite ends of Lake Erie can differ, so a local station is more useful than a lakewide average.
Check the Marblehead water-level station and current weather before relying on a familiar depth or fixed dock height.