Well, not to disagree politely, but I will. There is a substitute - If you're actually intending to be together, then stay together - don't one of you decide to go rushing off, and only think about it once you can't find the other one.

Both scenarios show a bit of a lack of thinking ahead - and had you been sligthly further out of reach, (so the whistle wouldn't be heard) you'd then be saying you'd have needed a two way radio, etc.

Similarly "interpreting it to mean you are in serious trouble" - if you're going to use an audio signal don't interpret - agree ahead of time and stick to it. (i.e. three blasts 15 seconds apart means trouble - answer that with one blast)