For underwater communication, nothing beats the ol' hit-tank-with-knife (or the slightly more sophisticated tank banger). https://www.amazon.com/s?k=tank+bange

For underwater communication, nothing beats the ol' hit-tank-with-knife (or the slightly more sophisticated tank banger). https://www.amazon.com/s?k=tank+banger https://www.amazon.com/s?k=tank+banger