Good advice from Ale and Tommy_Boy--that I never TAKE!
My view is every high end rep diver should be swim worthy OTB. I will generally put some silicone on the seals around the case back and crown and call it a day. I then shower, dive, wash to my heart's content. I have never had a watch flood on me, although I have had a couple have their crystals fog. If that happens, I take the caseback off--whip out my wife's blow dryer and get rid of the moisture. I will then close it up and give it to my watch smith to have it professionally waterproofed because there is usually an issue (pinched crystal seal etc) that needs professional attention. If I am having any modding done to the watch, I'll have it waterproofed and tested in the ordinary course of the work.
In fact, this morning, I took my new LMPO into a hot steamy, high pressure shower without any silicone etc. and no problems at all!! Anyway, here's to living dangerously!!!