Originally Posted by biffer
Pressure doesn't change when you're moving about. I'll see if I can remember boules law. At 30m the water pressure is 4 time greater than on the service.
We all live for the most part at 1 bar which is 14.8 psi at 30 m it's close to 60 psi. No amount of moving about will change that
Actually, it does. When you swing your arm underwater, you encounter resistance. That resistance is an increase in pressure against your arm. The seal in the watch or flashlight or canister experiences the same thing.
Oh, and as an aside, a green laser can be a pretty good surface marker without actually shining it on an aircraft. Swung around in a circle, it would make a pretty obvious focal point for the pilot to home in on.