I test all my water resistant watches by taking them in the shower everyday for 2-3 weeks, and I take HOT showers! I pour soap on them and push their buttons and rotate their crowns. If they survive, they're usually water resistant enough for my needs. I've tested this on watches from "water resistant" all the way through the 200M models. The only time one ever leaks is when a gasket is missing or bad, or when the watch isn't water resistant to begin with. My vintage Casio databank calculator survived about a week in the shower with soap and water, but fogged up in week two. It had a slow leak. It didn't say it was water resistant though.o|o|o|o|o|o|o|o|o|o|o|o|o|o|o|o|o|
I've swam with "water resistant" watches with pop off type backs.
I've taken this in the spa a few times and held it against the jet for 20 minutes!
http://www.casio.com/products/Timepiece/Classic/F105W-1A/
I've also taken my 50M twin graph watch, world time watch (50M version), and several other 50M version in the shower, pool, and spa.
I've shower tested my Seiko 5s too. No problems, and they're just water resistant.
These watches survived too:
http://www.casio.com/products/Timepiece/Classic/W201-1AV/
http://www.casio.com/products/Timepiece/Classic/W96H_Series/
http://www.casio.com/products/Timepiece/Databank/DB36-1AV/
http://www.casio.com/products/Timepiece/Databank/DB37_Series/http://www.casio.com/products/Timepiece/Databank/DB37_Series/product/DB37H-1AV/
http://www.casio.com/products/Timepiece/Sports/RFT100-1V/
And these had no problem:
http://www.casio.com/products/Timepiece/Sports/W727H-1V/
I got this one July 2, 2008, and I'm still testing it, but so far it hasn't leaked:
http://www.casio.com/products/Timepiece/Sports/W756_Series/product/W756B-1AV/
And of course, the G-shocks:
http://www.casio.com/products/Timepiece/G-Shock/DW5600E-1V/
http://www.casio.com/products/Timepiece/G-Shock/G2300_Series/
http://www.casio.com/products/Timepiece/G-Shock/G2500-3V/
http://www.casio.com/products/Timepiece/G-Shock/G7500-1V/
I like doing these tests when the watch is new because it's under warranty, and it SHOULD survive just fine. I'd be careful with new old stock, vintage, rare watches, family heirlooms, or anything with odd protrusions, like this:
http://www.casio.com/products/Timepiece/Databank/CA53W-1/
It says water resistant, but I don't trust it, it has rubber buttons sticking out.
(It's NOT a databank either!!!)
And I would NOT try unscrewing a crown while it's wet. Some screw down crown watches do
NOT have gaskets on the stem!!:-

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Every 6 months I retest my watches until they're about 3 years old. Then I just assume they're water resistant until they're 9 years old, then I assume they're NOT water resistant anymore.
I do these tests at home because if one fails I can take it apart and dry it out immediately, can't do that out on the lake or at the beech.
I don't take my watches a-showering everyday. The heat eventually ruins the gaskets, but the few times I do it over the watch's lifetime seems OK. Also, I think it's good to get extra gasket lube off. That stuff absorbs dirt and crud that I've seen ruin gaskets.
If you're worried about your 50M watch try getting a new one of these and see if you can get water in without doing something extreme like boiling it.
http://www.casio.com/products/Timepiece/Classic/F105W-1A/
Take it in the shower, Spa, and Spray it with the garden hose. Try getting it wet continuously for several days on end Don't worry too much, it's inexpensive. Try anything you're afraid to do on a watch you care about.