I'm not big on quartz watches, so I've never felt the need to have a caseback tool. But a few months ago I bought an older Heuer diver and wanted to put a fresh battery in it. To my surprise, when I popped the back from the watch, I discovered there was no caseback gasket. Nothing. Not even a remnant of an old one. So immediately I set out to pull the rest of the casebacks on the half dozen or so watches I have that I don't know for sure that are good to go. Found two more. One in a dive watch that I would not have thought twice about getting dunked.
In the last three weeks I have received four watches. A couple year old Glycine, a 60's Bulova dive watch, an Elgin dive watch and a fairly new Omega Seamaster. Of the four, only the Glycine had a gasket on the caseback. One was bought from a "professional" who I know did some work on it.
I have been fortunate that I have never had a watch take on moisture. But now I'm guessing that was just dumb luck. I have a couple of go-to watches for the boat or beach and I know they're well-sealed. I wonder how many watches I passed along to unsuspecting buyers who assumed, like I always did, that there was proper protection present.
From here on out, everything gets opened.