Which feature ensures that the Fox 40 whistle can be used underwater?

The feature that ensures the Fox 40 whistle can be used underwater is its pealess construction. This design eliminates the traditional pea found in many whistles, which can become wet and unable to produce sound effectively when submerged. A pealess whistle operates through a different mechanism that generates sound without the need for a moving part that can be affected by water, allowing it to maintain functionality even in underwater environments. This makes the whistle highly reliable for various applications, including marine use, where moisture is a factor.

While features such as waterproof design, lightweight material, and non-corrosive components contribute to the whistle's overall performance and durability, they do not specifically address the ability to produce sound while submerged as effectively as the pealess construction does.