Some rubber dive straps (like the rubber straps of a Panerai Submersible for example) have some sort of bumps in the strap. Some people find these bumps comfortable while others hate it, but very few know that it actually serves a very important function for divers.
When diving deep into the ocean, millions and millions of liters of water will push their weight on you. The deeper you go, the greater the pressure. Water pressure pushes its force onto your body, including the arms and wrists. Because of this pressure, your whole body, including your arms and wrists, get slightly compressed.
This calls for a crash course in water pressure and air volume. For every 10 meters you go under water, one bar/PSI gets added. Standard air pressure at sea level is 1 bar, so if you dive 50 meters deep, you have a pressure of 6 bar pushing on your body. Air volume also changes drastically when diving deep underwater. If you bring a balloon filled with air under water, the balloon will have shrunken to 50% of its original size after going just 10 meter deep. Go down 20 meters, and the balloon will shrink to 33% of its original size.
If just 20 meters of depth can have such a drastic effect on a balloon, imagine what 500 meters would do on a human body.
The compression on a person’s wrist can’t of course be compared to that of a balloon, but the compression can be enough for a steel bracelet to become lose and start wiggling and flipping around your wrist. When you are diving hundreds of meters deep in the pitch black depths of the unknown, the last thing you need is your most important tool to get loose and start dancing around your wrist.
Divers trust their watches with their lives. Imagine exploring a cave or shipwreck, and your most trusted tool where your life depends on suddenly gets upside-down and impossible to read.
The bumps in the rubber strap make sure your strap has extra stretch, so you can apply more pressure when strapping it on your wrist. If you stretch the strap to the maximum when strapping it on, it should stay snug around your wrist, even when exploring great depths. This is a feature steel bracelets don’t have.