My suggestion would be Holmes Bend Marina at Green River Lake. There's a campground on the road into Holmes Bend Marina, just a short ways from the marina/launch ramp. The Marina rents pontoons. They also have a beach area, right next to the launch ramp. Launch ramp is owned/operated by the COE ... and there's a $5 fee to launch.
campground is not affiliated with Holmes Bend Marina.
Campground website : https://www.recreation.gov/camping/h...O&parkId=73208
Marina website : http://www.holmesbendresort.com/