My feeling is that for a larger kit, bailout bag or car/boat kit, the larger whistles are fine. given the available room even a Storm whistle, a fairly huge chunk of plastic, might be the way to go. Large and in a bright color you can hang it on the outside where the bright color helps you find the kit.

For personal carry, where whatever you carry needs to be small enough to not get in your way and rounded/flat enough not to beat you to death if you fall on it, the Fox Micro would seem to be the better choice. Yes, you give up some loudness but being small and less annoying to carry your more likely to have it on you when you need it.

The Fox Sharx is an interesting compromise bigger than the Micro by quite a bit it is still a fraction of the effective volume of a Storm whistle. I don't think it displaces the Fox Micro because the Micro rides well on a lanyard around the neck or hung from the front of a life jacket or slipped into a pocket.Anything larger or bulkier won't do as well.

Given that the Fox Sharx is about as loud as the Storm it might replace the Storm whistle in some roles. Then again there is something very comforting about the relatively huge and bulky Storm. Using one you get the feeling that if a whistle will save you the Storm will do it.