what is the best whistle ,where folks buy it

The World's Loudest Whistle

http://wind-storm-whistles.com/

My pfd came with one already attached

When on the water, you want to have a pea-less whistle because when the type with peas get wet, their volume is greatly reduced. Arguably, one of the best on the market is the Fox 40
http://www.sunwavesports.com/sports_and_safety_p/10579.htm?1=1&CartID=0

I just took a mouthful of water and blew it through my whistle and continued blowing and it was just as loud as before.  Fox40 looks good, but I'll stick with my trusty Scotty.   

That's cool, I hear what your saying. Immersion over time can cause problems, a mouthful or splashes may not cause any issues.

I used to work in a job that included water rescue and they were recommended by some lifeguards and paramedic/firemen I knew, we tried them and switched, their tone and jamb-free ops were great.

Just having a whistle of any kind is good! 

This site I just found on the comparison of whistles is pretty interesting:  http://bigpigblog.com/tag/safety-whistles/  Seems many are only heard up to 3/4 of a mile to a mile?  I've heard that the human voice carries up to two miles over water on a calm day, so usually imagined that the whistle would be much farther. 

I also found this bit interesting: http://bigpigblog.com/2014/05/15/whistlemania-ii-the-iceman-cometh/   Not that I'm here to argue for or against pealess whistles.  Just sticking by what has worked for me for almost 7 years now, and don't feel like they deserve a bad wrap under the guise of a possibly perpetuated myth.

I give, Alain! 

I read your posted link test and looked around some more on the web. It appears there are strong divisions in sports officiating circles on the subject of whistles, their performance and shortcomings. I certainly don't want to perpetuate a possible myth, and I don't like to be mislead, either.

It seems it is a very personal and individual choice as to which whistle type/brand is best or preferred. Like we usually say when someone asks which is the best kayak - it's best to try them out and see what works for you. At about $5-$6 each, it won't break the budget to buy a couple different ones to have options and spares and maybe it's good to have more than one anyway.