Diving & whistles. The one time I really wanted to use mine, the sea state was such that I was either in a trough for half the time, or the whistle was full of water for most of the time, & I could barely get a sound out of it. My poor dive leadership had meant we'd surfaced 100 yards down tide of the boat, with a tide running, & 3-foot waves. I started carrying diving flares & a mirror after that.
In those circumstances a whistle was of no use whatsoever, although of course those circumstances were certainly not typical.