A whistle is one of my 'essentials'. All our kayak vests have a knife and big whistle. A whistle will cut through whitewater noise better than screaming your head off. A whistle will save your vocal cords when you call for help.
I can send morse code on a whistle and have!

However, Phat is right. Don't EVER separate unless you file exact flight plans with each other and even then, it's best to not separate out in the boonies.