It would appear that a watch claiming to be water-resistant to 100 meters or 330 feet should be sufficient for recreational scuba diving. Especially since those safe diving limits are set at 130 feet. So what's the deal?

First off, the water-resistance ratings provided by manufacturers are from testing done under controlled circumstances. Basically they indicate resistance to water penetration assuming there is no movement by either the watch or the water, at a particular depth. Also, they don't account for what happens if the watch is bumped or jarred.

Several factors are at work simultaneously on a watch while underwater. Pressure is the most significant. At sea-level there is one atmosphere (atm) of pressure on everyone and everything. That equates to 14.7 pounds per square inch (psi). For every 33 feet of depth, divers and their equipment are subject to one additional atm of pressure. Thus at 132 feet down, the pressure is 5 atm or 73.5 psi.

Furthermore, as a diver moves through the water, additional pressure is exerted. All this simply means there is significant force against the case, trying to compress it and force water inside. To withstand this force, a dive watch must be put together in such a way as to prevent any seepage.

The watch case itself must be strong enough to withstand higher pressures. This usually means stainless steel or titanium, although some polymers work as well. Something like solid gold may be a problem just because it's softer. Likewise with plastic, which may flex under pressure.

Seals must be used at each point where the case can be opened, and at other joints. This is primarily the case-back and crown (covering the stem). They may also be placed between the crystal and case unless a sealant is used. These seals, or gaskets are usually o-rings made of rubber or some other synthetic.

Every watch made for diving should also feature screw-down case-backs and crowns. A case-back that screws down tight against a seal works much better than any type of back that is pressed in. Same goes for the crown - it needs to screw in so it seats tight against the o-ring to prevent any seepage around the stem.

When trying to determine if a particular watch is suitable for diving, look for all of the features previously mentioned and for a water-resistance rating of at least 200 meters or 660 feet. That will be about 20 atm. Some watches will have a depth rating of around 500 feet and adhere to an ISO standard saying they are made for scuba diving. Those are suitable as well.

The apparent overkill on the depth rating means the watch should hold up fine underwater for most normal diving conditions. The additional durability will also help them survive a few bumps, as long as they aren't jarred so much that the case or seals give way enough to allow moisture to squeeze inside.

This also brings up the subject of care. Subjecting your watch to quick hot and cold temperature changes make it much more likely to leak due to rapid expansion and contraction of the metal. In addition, chemicals like chlorine are hard on seals.

Take time following any saltwater diving, or swimming in a chlorinated pool to rinse off your watch with fresh water. Rotate the bezel also to flush out any salt and sand. Taking an extra couple seconds for rinsing will make a big difference in terms of longevity, same as it does for the rest of your dive gear.