Have you tried looking here -
Home - The Seaplane Pilots Association at all the listings? I know of 3 - Manitowoc, WI, one in MInnesota and another in California, all are good, and all have an abundance of beautiful scenery.
I did mine with Caledonian Seaplanes about 12 years ago on Loch Earn, and assorted other lochs. It was at this time of the year, fabulous weather and just a fantastically fun weekend of flying.