Found a few more whilst looking on flightsim.to and added to the list. They are:
8MI2 - Elberta Seaplane Base, Michigan by MulberryWing. Quite a nice little seaplane base. It’s fictional but based on the site of an old ferry harbour and fits in quite nicely. The Great Lakes really needs more places for the floatplanes to live! MulberryWing also has a number of historical/fictional old school style airfields in Michigan that are worth checking out.
NY9 - Long Lake Helms Seaplane Base by ADKAIR83. Pretty decent seaplane base in Upper New York with some improved coastline as well. Nice spot to fly around. ADKAIR83 has a few more strips in Upper New York including ADK2 - Elk Lake Lodge Airstrip which has a water runway but no parking docks. There’s a bush strip with parking spots so I guess you could drive your amphibian over a bit of marshy ground to get to the water!