You are correct sir!!!! All you need is enough foam in the tube to keep the water from rushing in and filling it up. When you seal the water out , the trapped air will help keep it afloat like a bottle with the cap on. Make sure you put foam in both ends. I used a piece of hose to shoot the foam deeper into the inner tube also. Don't use too long of a hose, that stuff gets sticky Quick and will stop up the hose. :-)

