Like you, I can't fly in the back yard because I live on a small weedy bay surrounded by trees. Also, there's a "no engines" rule in the lake. The rule was obviously created to apply to boats, but I respect the intent of it.

My club flies from the bank of a manmade lake. Our vantage point is on a peninsula, so we have a lot of latitude setting up to align with the wind. We stand about 6 feet above the water level.

I learned to fly floats from a boat. I had an 18' runabout with a walkthrough windshield. It was ok but the freeboard was so high it was hard to grab the planes over the bow. I also flew from a 12' coleman plastic rowboat. We'd go out into the middle of the Tennessee River and fly from there. The river is nearly a mile wide since it's dammed up, and there was lots of room to fly.

If you have trouble with waves rocking the pontoon boat, suggest you go out on a weekday afternoon when it's quietest, and also sit down to fly.