Underwater alarm?

Does anyone have suggestions for an underwater device to signal swimmers? We'd be using it from a kayak, so something along the lines of a "clicker" might work the best. Any ideas? Thanks. JB I'm not sure what you're trying to accomplish with this, but my most un-ignorable method at the pool involves banging on the pool ladders with a wrench. Much more effective than yelling or whistling. So try a short length of pipe and something to bang with, should be able to handle that in a kayak. Make sure the pipe is in the water when you bang it!

Does anyone have suggestions for an underwater device to signal swimmers? We'd be using it from a kayak, so something along the lines of a "clicker" might work the best. Any ideas? Thanks. JB I'm not sure what you're trying to accomplish with this, but my most un-ignorable method at the pool involves banging on the pool ladders with a wrench. Much more effective than yelling or whistling. So try a short length of pipe and something to bang with, should be able to handle that in a kayak. Make sure the pipe is in the water when you bang it!