Here’s an idea that many people have enjoyed: an inverted bowl or half gourd floating on water can produce a satisfyingly deep, clearly pitched tone when struck with a mallet. The tone comes from the air trapped beneath the bowl, and the pitch relates to the volume of enclosed air. I found that the bowls of thin pressed wood shown here work quite nicely. I added styrofoam floats to the sides of the bowls to prevent them from tipping sideways into the water (as they otherwise sometimes do).
In addition to inverted floating bowls, at the end of the sound clip you can hear wavery bell-like sounds of metal bowls floating right-side up.