DearDiveBuddy.com

A "wet breath" is defined as taking in water together with air through the regulator while you breathe. It can be just annoying if it's a bit, up to making the regulator useless, being forced to swap to your alternate air source. To go a bit deeper into the reasons of a "wet breath". The vast majority is caused by the diaphragm, mouthpiece or the exhaust valve. All concern the second stage.

Reason #1: The Diaphragm.
Not a rupture is most likely to happen, but a displacement. Many diaphragms are held in place by the cover screwed down and keeping the edge of it between the cover and the regulator case. Usually there is a thin washer (ring) between the cover and the diaphragm. If this one is missing, mostly due to "maintenance" by non professionals, the diaphragm can pop out easily during the dive. Most likely, when jumping in or when you are under heavy workload and breathe in stronger, so when you need your air the most.

Reason #2: The Mouthpiece.
A mouthpiece torns most of the time on the upper side where it joins the hard part of the regulator. It slowly works its way through the silicon. Pull the mouthpiece slightly to make it visible. The point is that the torn can be covered by the lips that seal it of. Changing the position can then uncover the torn and water runs in. The mouth and lips becoming more tense as you are in distress that only makes it worse. Or not knowing the cause someone with big lips tries it and goes "there's nothing wrong, it's working fine for me".

Reason #3: The Exhaust Valve.
Some (not all) regulators with downward exhaust tee can even be used upside down, without substantial amounts of water leaking in. Very nice if you swim backwards and then tip your head backwards to see where you are going (as practiced by the dive guides). This is due to a soft and good sealing silicon exhaust valve. With normal use any problems with the exhaust valve are unlikely.

Periodic preventive maintenance includes normally a renewal of both the mouthpiece and exhaust valve. Diaphragms are only changed if in bad shape.

Although it sounds strange, you check for a "wet breath" out of the water. Just suck your reg with the tank closed and no pressure. If you take air in it's wrong (during the dive this would be water). You should "hear" the membrane being sucked inside, without getting any air. This check is more important than the one you see most people do, checking if they get air with the valve open. You do that anyway before you jump in.

Thanks to Dave "the Cave" (David Summerfield).