To be legal, kayaks (canoe, etc.) need to have C.G. approved PFD and a whistle (or air-horn). The Nexus Pea-less Whistle is and has been the standard marine grade (pea-less) whistle for wearing on a PFD. It meets the US Coast Guard requirements for kayaks.

A cork ball (pea) in a whistle can swell in water and jam making the whistle stop working, so for boating you need a "pea-less" whistle. Kayakers generally carry their whistle on their PFD. Wearing a large whistle on a PFD gets in the way during re-entry rescues, and if you attach a whistle to the zipper pull on your PFD the whistle may catch on something and break the PFD zipper. So we recommend storing your whistle in a pocket on your PFD, and for that you want a small whistle. The Nexus whistle has a hole in the end so you can add a short loop of cord to facilitate attaching the whistle to a "D"-ring or strap in your PFD's pocket (most kayaking PFD's with pockets have "D" rings inside the pockets for this purpose).