There are several ways for salt water to enter your first stage. One is the result of a damaged cylinder valve outlet o-ring. When the o-ring where the cylinder valve and regulator first stage mate is damaged due to heavy use (especially rental tanks) a small amount of water can get drawn into the system while breathing. A telltale sign of such a leak includes hissing before the dive when air is turned on. Rotating the regulator may stop the leak, or not. Underwater this appears as a constant stream of fine bubbles behind your head and you'll probably be annoyed by the 'fizzing' sound.

This is usually a low-volume flood that corrodes the inlet filter. In worse cases it can flood the pressure gauge. In time, corrosion will cause first stage failure, namely a leaky second stage or octopus.

You can prevent this by inspecting the valve outlet o-ring before use, ideally at the dive shop because they should have o-rings, and replacing the o-ring if it isn't in perfect condition. Shops that really care may include a spare with the dust cap or Sherwood burst disc nut. Keep a few spares and a sharp tool to remove the old o-ring in your dive saver kit. If you own tanks, I recommend switching to urethane (cream-colored) o-rings. They are highly resistant to wear and last much longer. Trident R014-S for standard Thermos or Sherwood valves or R112-S for Pro-valves and DIN regulators.

Keeping your alternate air source (octopus) secured near your chest is important to protect your gear and the environment, in addition to making it easily accessible in case of emergency. However there are situations when the best care does not keep sand out. In my case this happens after a dive at the Halona Blowhole, not to mention Rescue Diver training at Waimanalo. If you are unlucky, sand gets stuck to the regulator's low-pressure (LP) seat and causes a slight but persistent freeflow.

Aside from taking a few strong breaths or depressing the purge button a few times in hopes of clearing the LP seat, there's nothing that can be done to safely fix it while diving. In fact, turning off the gas supply and fully depressurizing your regulator while submerged will cause a catastrophic flood and ruin the pressure gauge. Therefore this can only be done out of the water. The following technique will allow freshwater to enter the system where it should not normally be, so I do not endorse it as a 'good practice'. However, if you are on a dive expedition without access to a trained scuba technician, this may solve your problem.

After diving keep the regulator mounted to the cylinder with gas supply turned off and depressurize the system. Fully submerge the leaky second stage in a container of freshwater or use a water hose to flush the second stage via the mouthpiece. Contrary to what you were taught in Openwater Diver class, depress the purge button a few times for one second intervals to allow a small amount of water to enter the system. Turn on gas supply and purge second stage several times, blowing out the water and hopefully the contaminant. This frequently solves the freeflow problem but there's a chance that sand is too deeply embedded into the LP seat, or the sealing surfaces have been damaged, requiring service or replacement. At that point the system must be tested and probably overhauled.

There are other methods to fix this problem but must only be done by a trained service technician to prevent damage, voiding your warranty, or personal injury.

If you are lax when it comes to after-dive cleaning, there's a good chance large salt crystals may form and corrode any metal parts or embed themselves on the LP seat. In this case I'll expect to see you sooner than later, so treat your regulator with proper care. Your life literally depends on it.