What two substances can swimmers use to signal for help?

Swimmers can effectively signal for help using waves with an arm or a whistle. Waving an arm is a widely recognized signal that indicates someone is in need of assistance in the water. It captures the attention of lifeguards or other nearby swimmers who can respond quickly. Similarly, a whistle is a common signaling tool that can pierce through ambient noise, making it easier to alert others to an emergency situation.

In contrast, utilizing floating devices and lights is more about providing visibility or buoyancy rather than directly signaling for help. Flares and signaling mirrors are specialized tools that, although effective in specific circumstances, are not commonly available to casual swimmers. Lastly, using large rocks or poles is impractical and may not effectively communicate a need for help in water emergencies. The distinction of using simple and recognizable methods, such as arm waves and whistles, makes them the most effective and immediate ways for swimmers to signal for assistance.