Amazon カスタマー 5 It easily noticeable even under water (probably not likely when in deep waters) and the screw cap has several O rings that would help prevent water from seeping through the threads between the housing and the cap.Only one feature that would be really cool to have....if it would be rechargeable using USB charging!Other than that the strap that comes with the product is useful for the flashlight to be strapped down to a wrist or to anything that that one would want to strap the flashlight onto to something.