Would it be okay to swim laps our shower with the Mark II reissue? I know it waterproof to 10 bar, but the Moonwatch is waterproof to 5 bar, and most advise not to swim or shower with it.

 

100m is generally assumed to be sufficient for light swimming and showering but as a precautionary measure I wouldn't recommend it for a chrono without screw down pushers/crown. But to each their own, and I've witnessed first hand and via the intertubes plenty of people who've taken their 50m rated Speedies for a dip and/or shower with no ill effects.
However if you really must there are two guidelines to follow just for added safety:

-You MUST have the watch pressure tested to see if it indeed is 100m / 10bar. Also, have all gaskets and seals checked. This will ensure that the WR rating is intact and that the watch has the greatest possibility of surviving the mentioned activities.
-Showering and swimming should be ok, but make sure you rinse the watch thoroughly (although NOT violently) afterwards. Some bath and body care products have ingredients that can adversely effect the condition of the all-important seals/gaskets, and same for pool/seawater. This is true also for any watch, even dive watches rated to 2000+m/200+bar.

 

I wouldn't take a non screw down pusher Chrono swimming unless it's a POC.

One accidental knock on a pusher and you've lost WR.

 

Omega indicates that a 100m watch is good for...wait for it...100m! Certainly the advice is sound to get it pressure tested regularly, and I'm never a fan of showering with a watch on, but it should take swimming just fine. I wear a Speedmaster a lot, and can't say I've ever inadvertently pushed the pushers on one, but I guess it's possible.

Cheers, Al