An emergency whistle is a very useful signal device. While not as far ranged as other devices, the sound of a whistle is unmistakable and is particularly effective in environments where visibility is poor. A signal whistle is often generally used to get the attention of nearby rescuers that are already looking for you. A proper whistle should be pea-less as whistles with a pea are used for other purposes and their sound could be ignored.
A plastic whistle is light enough to put on your keychain as part of your Every Day Carry.