I bought the loudest whistle in the world to carry when I am on water or somewhere far fishing. They said you can hear it up to 2 miles if there is no building on the way.

 

The Storm All-Weather Safety Whistle is the loudest whistle on the market

I got a little yellow one at REI.  I tested it once and my ears hurt for 15 minutes 

I've been using a Fox 40 for years.  Got one on every PFD except for the fishing one.  They're plenty loud (never tested beside this one), but you can't hear it for 20' on Little Falls if the gage is over 4 1/2' or so.  A whistle might be useful in the woods or something, but in a rapid, you just end up relying on vision so much more.

I hope it comes with ear plugs. Thanks for the tip!