All is not what it seems with watch water resistance, you will not find a watch that claims to be waterproof. This is because no watch can be guaranteed totally waterproof.The different levels of water resistance as expressed in meters is only academic. They refer to the depth at which a watch will keep out the water if both watch and the water are static. So for example if you have a water resistant watch and you dive into a swimming pool the pressure on the watch at the point of impact with the water will be many times greater then if you were just placing the watch into the water.
A gasket or O ring is used to make a watch water resistant these can be made of rubber ,nylon or Teflon and form a water tight seal at all the joints including the crown (the winding stem), case and where the crystal glass meets the watch case. Another aspect of water resistance is the back of the watch, a screw on back is far superior to a push on back. DepthThe depth that a watch can be used at is a bit confusing a watch marked as a water resistance of 10 meters you would initially think that you could safely use that watch in a swimming pool as long as you do not go deeper then 10 meters. This is not the case, as a watch that is water resistant to 10 meters is designed not to be used under water and can only withstand light splashing of water. Even a watch marked as water resistant to 30 meters should not be worn while swimming in a deep pool. For swimming you are looking at the minimum of 100 meters water resistance to safely use this in a swimming pool.Things that affect water renitence in watches You should not use you watch in a hot tub or sauna, as the heat will make the metal within the watch expand at different rates then the gaskets this can allow water to get inside your watch.Water resistance chart
Water-resistant to 30 meters, Will withstand splashes of water or rain but should not be worn while swimming or diving.
Water-tested to 50 meters, Suitable for showering or swimming in shallow water.
Water-tested to 100 meters, Suitable for swimming and light snorkeling.
Water-tested to 150 meters (500 feet). Suitable for snorkeling.
Water-tested to 200 meters (660 feet). Suitable for skin diving.
Diver's 150 meters, suitable for scuba diving.
Diver's 200 meters suitable for scuba diving.
The water resistance markings on watches in my opinion are misleading and should be redesigned to tell you the consumer what depth you can actively take your watch to while doing normal everyday activates. Without knowing a little about this subject having a watch, marked 30 meters would make me think that I could use this in a swimming pool without any worries.