No particular reason other than to see what they look like.
That's why people have posted underwater photos of dive watches—simply to show what they look like underwater. Other than that, why would anyone feel the need to post photos of dive watches underwater? We know they can go underwater without any problems.
But we don’t know if water resistant watches can, despite what their water depth rating suggests. So it would be informative for collectors to know this in case they want to buy a particular model.
So any underwater photos of such watches would demonstrate they they are indeed water resistant and in some cases even "dive-resistant".
A lot of talk around the forum lately about 3 ATM watches being perfectly fine for swimming, but based on this thread there are very few people willing to back it up with a picture of their watch actually in use while swimming.
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