Watchco SM300 and water resistance

I have a mate here in Oz who owns a Watchco build Seamaster 300. For those unfamiliar with Watchco - a bit if background: Watchco were an Omega service centre in Australia and held a large parts inventory. In the early 2000's, they started building new SM300, SM120 and SHOM's from brand new replacement parts and period correct movements. Omega never had a problem servicing these watches. This watch he owns is possibly around early 2000s not sure when they stopped producing.

He has recently had the watch serviced by a very experienced Omega repairer. My friend wanted the watch pressure tested but the repairer has stated that his newly serviced watch would most likely pass a pressure test, but even if it did even if it did he couldn't guarantee that it would keep water out "...no watch is guaranteed water resistant regardless of if its a dive watch or not" my friend rightly asks the question, what is the point if a dive watch that is not guaranteed to keep water out even if it passes a pressure test. In this case, perhaps age has something to do with it, but I would have thought it would be more to do with gaskets.

In an attempt to help him (and me) understand I would be keen to hear your thoughts on this? Are watches guaranteed water resistant from the factory?

Thanks in advance

I have a "watchco" and I have snorkeled and dived with mine and have not had a problem at all. A proper rinse once out of salt water is all. I get my divers pressure tested at the watchmaker and if necessary replace gaskets needed to ensure water resistance.

Watchco 8 months or so. Submariner at last service 4-5 weeks ago. Both have been in ocean numerous times and have not ever worried about them. Owned the sub for 9 or so years and have not had a problem at all.