I first saw the idea for this toy boat in the Japanese animated movie, "Ponyo". I found that actually this "Pop pop" or "Putt Putt" boat toy dates back about one hundred years. The picture above most clearly illustrates the concept for the boat. The candle heats the water in the tube. It turns to stream and gushes out both ends, which creates a vacuum, that quickly sucks water back through both ends and the cycle repeats. The one in the movie and most toy versions use a flattened diaphragm boiler. Which creates more of a putt putt sound and is a little stronger. But the home made coil version works well, too.
Take some thin copper or aluminum tubing which usually comes in 12 inch lengths. Wrap around a dowel or battery and make 2 or 3 coils. 3/32 inch or 1/8 inch size tubing are recommended. Attach the motor to your boat with both ends sticking in the water. Use a tea lite candle as a power source. The boat's hull can be made from almost anything that floats; balsa wood, Styrofoam, a tin can or an old plastic dish. I've included pictures of various kinds. There's also a great website all about pop pop boats with history, physics, and tutorials. Check it out and have fun.
Pop Pop Pages