I bought an inexpensive Chinese quartz wristwatch for my grandson, about $11 inc post).
Importantly it claims to be water resistant to 3atm (3bar), and the pics given on eBay clearly showed a screw on back (even weeks after becoming aware that is deceptive and misleading).
In fact, the watch that arrived had a snap on back, and that is inferior to a screw on back in terms of water resistance. I would not have bought it with a snap on back.
First test was of its time keeping, and it is stunningly good, a few seconds a month and each second is consistent, it does not seem to use inhibition of some second ticks, but is regulated for each second tick.
Second test was water resistance.
Above is a clip from a video of a pressure test of the watch at 3bar. Bubbles can be seen streaming from the case back just above the crown.
On the basis of non-compliance with the claim of 3atm water resistance, the purchase price was fully refunded.
Immediately on failure of the pressure test, I removed the back and the cause was obvious. The o-ring seal that belonged in a recess on the back was loose inside the case. The probably reason for this became obvious when trying to fit the back after drying the watch out and greasing and properly fitting the o-ring. The back was very hard to snap on with the o-ring in place, in fact I needed to use a case press to close the back. It would seem that on the production line, it was easier to throw the o-ring inside the watch (no left over parts) than to close the back with the o-ring in place (too hard to do by hand with the new o-ring).
I have not bothered to pressure test the reassembled watch, snap on backs are not conducive to reliable water resistance.
Had the watch been made with a screw on back as pictured in the eBay item description, it would have been easy to properly install the back with o-ring… but they have apparently elected for a cheaper construction, then failed to assemble the back to obtain any level of water resistance.