If you just bought a watch rated “50M”, you would be forgiven for assuming that it is waterproof to a depth of 50 meters. So can you go diving with it? No way…
Ratings such as 30M, 50M, 100M and 200M indicate watches that are in some way water resistant, but are unsuitable for scuba diving. If you want to take a watch diving, you absolutely need to get one that is specifically rated for diving.
For most people a rating of 50m will be adequate for recreational swimming, and a rating of 100m will be suitable for other water sports. Here's a more detailed breakdown of what you can do with “water resistant” watches:
|Walking in the rain||y||y||y||y|
|Swimming in still water||n||y||y||y|
|Professional water activities||n||n||n||y|
|Deep diving without scuba||n||n||n||y|
Even if you follow this table, you're not supposed to press the buttons while the timepiece is wet. To maintain the water resistance you must avoid taking the watch into saunas or hot tubs, and must have the seal renewed whenever the battery is replaced.
The testing of watches for water resistance is described by ISO standard 2281. The standard only requires that a sample of watches is tested, and the sample is only tested with a static pressure (which explains why you can't actually go diving to 50 metres depth and expect a 50M watch to remain waterproof). Furthermore, not all watch manufacturers use ISO 2281.
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