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