In short, YES! When computing flotation there is a little bit of math involved. Here's how it works. When you place anything in the water, a giant hunk of steel or a tupperware container, that thing is bouyed up by the amount of water it displaces. Water weighs 8,34 pound per gallon. The are 7.48 gallons of water in 1 cu.ft. 8.34*7.48=62.38 Therefore 1 cu.ft. of water weighs 62.38 pounds. Conversely, 1 cu.ft. of air contained within your Tupperware container will float 62.38 pounds of something. Let's say you weigh 150 pounds and the framework of your boat weighs 50 pounds. That's 200 pounds. 200/62.38=3.2 This means you'll need 3.2 cu.ft of air space (in your rubbermaid) to float "in the water." Let's look at a larger example...the Titanic!. It displaced 60,000 tons of water. (60,000*2,000 pounds per ton)=120,000,000 pounds/62.38=1,923,693 cu.ft. So the amount of space under water for the short time the Titanic was floating was just over 1.9 million cu.ft. That's just the part that was under water. Don't forget, there was even more above water. This number is actually a little high because the salt in the water helps the bouyancy a little. 5 gallon soap buckets are cheap (often free), waterproof and easy to work with.