If your covers don't work, do something about that. Fun2dive's and mine are all DINs with screw-down delrin covers - they're solid. Just don't push the purges on the second stages when they're in the tank. Scubapro has a decent screw-down cap for their yoke regs that seals very well. There are other manufacturers that don't do nearly as good a job.
A dunk with a tank on it is very nice, and optimal (while the air is on) Now figure out how you're going to do that at most private homes. Not realistic.
The problem is that if you don't get the salt out of there you will get corrosion in a piston reg where the large end of the piston rides. Once this eats through the chrome plating the reg will no longer seal, as the O-ring's sealing surface has been destroyed - and that part is expensive enough (the main body piece) that once that happens you may as well throw it out.
The same thing happens to seconds with a metal airtube or other metal parts under the cover. If salt is allowed to remain in there it will corrode those pieces; that corrosion is often invisible until you have it torn down for maintenance, at which point the tech goes "eeeeeewwww!"
Most times a corroded second can be salvaged though, although in severe situations the crown may need to be replaced if its made of brass (that's a fairly cheap part though)
NO balanced piston is suited for cold water, unless you add an "environmental kit" (basically, you fill the environmental chamber with grease, so no water can get in there, then put on a trim ring to keep it from oozing out all over everything - it still does ooze, just not as much) That's a hellacious mess to clean up on every annual though, it will leak a bit in your gear bag or on the shelf, particularly if it gets hot (e.g. you leave it in your car in the summer) and if its silicone grease using the reg with Nitrox is probably a really bad idea (oxygen and silicone grease get along
real well...) If you use Christolube for the grease (compatable with elevated O2s) you're going to end up using $30-40 worth of it for each time you overhaul! That gets expensive FAST. Of course in warm water none of this matters; reg freezeups are pretty tough to provoke until the water is colder than the low 50s. The problem with a balanced piston is that by definition the outside water has to get into the reg and bear on the back side of the piston to provide the balance force - that water can freeze, and if it does then the piston can bind up and be unable to move. The result is usually that the reg stops sealing off the high pressure and you get a massive freeflow, although a lock-up (no gas delivered at all!) is theoretically possible.
The MK10, 10+, 20 and 25 are very close cousins. The 10+ onward all use the same seat, although SP doesn't tell you that and they've made small changes in the design over the years without being real forthcoming about exactly what they are. The 20 and 25 have a "composite" piston (there are several iterations of this around in an attempt to control freezing - see above - it doesn't work and never will) and a pair of fiddly little plastic bushings surrounding the high pressure O-ring instead of the O-ring riding directly in a machined groove in the body (MK10, 10+) I've been told that the reason for the change was that technicians were being intemperate with tools while removing that O-ring during overhauls, scoring the groove, which then destroys the body as the HP O-ring will no longer seal. The cynic in me also says that SP developed the 20 and 25 because the original patents that were on the Mk 5/10 series ran out and there were a dozen clones of them suddenly available, so they had to do something to differentiate themselves even if it was all marketing-speak.
I prefer the 10s and 10+s over the 25 because the 10s and 10+s are dirt simple to overhaul and have fewer fiddly little parts which IMHO do nothing of value, yet they perform just as well. They're also really easy to find at a nice price on the used market. Finally, the overhaul kits for the 10s contain
three seats, and depending on what you want the IP to be you can almost always use two of the three in a given reg, so you get two overhauls out of one kit (O-rings are generic and available from various places.) This makes them less expensive to maintain over time.
No piston reg is appropriate for cold water (under 50-55F) - for that you want a sealed, balanced diaphram such as the Apeks DS4. A sealed, balanced diaphram design allows no water inside the regulator at all, and thus is immune to freeze-ups so long as the gas in your tank has a sufficiently low dew-point.
(This does not mean that the
second stage can't freeze. It can and that's independant of the first stage design, of course. But most freeze-ups in cold water are actually first-stage freezes.)
Hope this helps.
