Bit of an update: I wanted to check if my PO was water-resistant, so I took it to a watch-smith who I've used over the years for battery replacement and pressurising of an old Seiko (he also sized the SS bracelet on the PO for me the day I received it) and told him I just wanted to check if it's OK for swimming, showering, etc. He came back and said it's absolutely fine - no leaks at all. He didn't tell me the degree to which it was tested so I'm guessing at least 5ATM as I mentioned swimming with it on - maybe I'll ask him when I take my next rep in.....

Five quid well spent, I reckon.