I've always been taught to keep a whistle on me in case of emergencies - it makes sense since it's small and lightweight but could really help in an emergency.
However, something I've never really thought about much is, are they pretty much all the same, or are some more desirable than others for various reasons? I'd assume metal ones are preferable for durability, but are there any other characteristics that should be taken into consideration apart from that?
The obvious thing to consider would be how loud the whistle is. The loudest I've found is the Lifesystems survival whistle which, at 122dB, claims to be "the world's most powerful whistle."
I'm not sure I agree with choosing metal whistles, as in freezing temperatures they would be quite uncomfortable to use. As regards durability, the plastic whistle I bought about 20 years ago is still in good condition.
The best whistle is one you don't forget. The last two packs I've had included whistles built right into the chest strap clip.
You should look at scuba diving stores. They have whistles that are so loud they put a BIG warning on the box that tells you not to blow them indoors because they can damage one's hearing.
If you do get one of these heed the warning. Your ears will be ringing if you blow it with half effort in a hotel room.
Note: These pea-less whistles work well underwater. So, you can imagine how loud they are.
... what qualities ...?
See the specifications for SOLAS accreditation, ISO 12402.
5.2 Whistles Whistles shall be non-metallic and robust in construction, free from all burrs, and shall not rely on any moving part for the production of sound. They shall comply with the relevant material requirements as specified in ISO 12402-7. The attachment and the cord of the whistle shall withstand a static load of (200 50 0 + ) N when tested in accordance with 5.4. Three specimens shall be tested by being blown as hard as possible by a subject of between 20 years and 30 years of age and free from all known impairments to pulmonary function, in an outside and open area during calm clear weather. The sound generated shall be shown at least at an instant to have exceeded 100 dB(A) measured at a distance of (5 ± 0,1) m directly in front of the whistle. The predominant frequency shall be (2 ± 1) kHz and shall be multi-tone. The whistle shall also be shown to be capable of producing sound in air immediately following immersion in fresh water. The whistle shall be attached to cord or line of a length sufficient to permit its use, which shall in turn be attached securely to the lifejacket or buoyancy aid. It shall be stowed on the device in such a way that the performance of the lifejacket is not affected, but so that it can be removed for use with either hand of the wearer, and can be stowed by the wearer. Whistles complying with this part of ISO 12402 shall be marked with an exclusive permanent identifying marking that can be traced to conformance with this part of ISO 12402
