It's common recommendation for vintage watches
Hi, yes I have been told this for other Omega watches as old as your MkII.
In the past few years I had my MkIV and a 176.0012 overhauled in Bienne and though they were both handed back to me with a pressure test result sheet that confirmed their waterproofness, they recommended that the watch not be worn in water.
I asked why and they said for so old watches they do not take any responsibility whatsoever in spite of changing the relevant parts, so they issue the warning as a disclaimer in the first place.
On the other hand, I never have worn any of my 'nice' watches in water, so I did not mind that much, but I can see it may be annoying to others.
Cheers.
Fabrizio