In preparation for this year's Camp Tipsy, I've started collecting buoyancy at my office. We go through a bunch of Vitamin Water (and bottled tea, orange juice, etc) each week. According to WolframAlpha, each empty bottle of Vitamin Water provides provides 1.195 pounds-force of buoyancy (or 5.315 Newtons) .
Step 1: Get a bunch of empty bottles with caps. The photo above is about 3 weeks worth.
Step 2: Build a big box. Like a coffin. 2x4s and plywood work fine. Angle the front a bit for that boat-like profile.
Step 3: Put the empty bottles in the box.
Step 4 (optional): Cut a hole in the box. It doesn't matter. As long as the bottles don't escape.
Step 5: add an outrigger for stability
Step 6: Toss it all in the lake. You've got "boat".