simple thing to tuck into your lifejacket pocket... i hear people discuss knives more than any other safety gear, but this is my number one...

http://baltimore.cbslocal.com/2018/05/23/4-safety-whistle-saves-life-stranded-md-kayaker

I agree, John. We used to run a 24 mile long beginner distance race called 'Round The Bay race here on Maumee Bay at the west end of Lake Erie and had a sailor rescued by Matt and Laurie Bounds because they heard a whistle blowing before they saw the guy in trouble. Maybe Matt will comment on what happened as I am sure he remembers it better than I do since he was involved. We made the whistle mandatory for racing so we recognized the rescue at the awards partly to emphasize the point that it really can be critical to have even cheap and simple emergency equipment on board and on person. I keep one attached to my life jacket.

yes.

Also on my CamelBack for mountain bike riding.

Hmmm...looking at your avatar I wonder, is that is a camelback? The front is called a cameltoe. And this is your safety device?

Agree with Mike - the guy was singlehanding on a Taipan, flipped and became separated. We saw the boat on its side, but nobody on it. We started scanning the area, but the sea state was large enough that we didn't see him until after we heard the whistle. Picked him up and transferred him to the USCG who rolled up just then. That was one scary race, but it convinced me to always carry a whistle. Doesn't have to be special. Just loud.

Don't leave home without it!

There are lots of whistles to choose from but LOUD is what you need.
I recommend a Fox 40 Classic. Plastic, pea-less and LOUD! Ouch, it's loud and doesn't take much wind to be LOUD.
It has 3 different tones which create a non-natural dissonant tone.

Did I mention it's LOUD?

I keep a fox 40 in my vest and it was my go-to whistle in my decade of pool life-guarding.

I carry Randall Hunter model knife on sailing but it's pretty old, which one should I get next?

No lifejacket is complete without a loud whistle, good knife and charged VHF radio attached to it. I spent a night (alone) on the bottom of a dismasted and turtled Hobie 16 once. Not something you ever forget, if you're lucky enough to survive it in the first place...

Mike

I used a whistle once when trying to clear some room in the surf so I could beach (with no land side help to clear the way) and I couldn't get anybody's attention with that thing. That experience soured me on the value of a whistle and I continued to carry cheap ones just to meet the letter of the law. I have three different life vests that I use for different occasions and one didn't have a whistle on it so, based on the recommendation here, I bought a fox 40 for my other vest last week. Holy hell is that thing loud. That's an amazing whistle - it made my ears ring and then some. I'm getting one for all my other PFDs and my boat kit and ordering some for my club to keep on committee boats...spend the extra $6 for that Fox 40 over a cheapo - it's most definitely worth it. All whistles are most definitely not the same.

Here's the Fox 40 Sonik Blast CMG item I bought: https://www.amazon.com/gp/product/B004UA66KO/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Thanks Jake - just ordered one