Obviously carrying a whistle is sensible, but has limitations with wind and the requirement for someone to be listening and willing to act.
I was once told a story of a Sea kayak leader assessment being carried out under the skye Bridge. The coastguard had been informed that people would be out of their boats swimming and whistles ect would be used.
Usual scenario games, sea kayak party split, some out of boats, one injured. Assessor ask the assessed what they would do. First answer was raise the alarm. Ok how would you do that? Candidate looks around and sees pedestrian on Bridge, and answers that protocol would be to put out a VHF Maday, but in this instance they would blow on their whistle to alert the pedestrian who would call the coastguard. Which would be quicker and allow them to get straight on to regrouping the party and organise putting people back on boats.
The assessor said ok let’s see if that works, and whistles………the pedestrian just carries on walking.