To be legal, kayaks (canoe, etc.) need to have C.G. approve PFD and a whistle or airhorn. As its name implies, the JetScream Whistle is really loud (just blow hard to get the rated 122 db), and it's a marine grade (pea-less) whistle for wearing on a PFD. It exceeds the US Coast Guard requirements for kayaks.
A whistle with a cork ball (pea) can swell 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 JetScream whistle has a hole en the end and comes with a short loop so you can simply add a few inches of cord to lengthen its tether to facilitate attaching the whistle to a "D"-ring or strap on your PFD (most kayaking PFD's with pockets have "D" rings inside the pockets for this purpose).