That corrosion kills gauges fast in my part of the world (tropics). I have found putting way too much silicone (Or Tribolube) on both the threads and the swivel to be one only effective way to slow it down, since as you have noticed seawater otherwise has access. Lubing the threads acts as a physical barrier.
the old HP swivels were better at keeper out gunk, and keeping out corrosion from the O-Ring but they would also end up corrosion welding themselves to the mating surfaces eventually. Most gauge makers switched to the smaller shouldered HP swivels with the centerplate but they can throw the o-rings off the reduced shoulder (usually on the expensive gauge side, and are far less effective at protecting the O-ring structurally. Before the swivels would weld themselves into place from corrosion and the gauge would die, now the swivel can always be removed but you can end up with a dechromed O-ring interface area inside the gauge which leaves the gauge useless anyway.
(Keeping rental fleets running in the tropics taught me that big vats of storage water, with just the right amount of vinegar to soak away daily corrosion overnight every night is the surest way to protect gear long term gear loss from corrosion. We kept a 30 gallon storage tub with a couple of squirts from a squeeze bottle of vinegar that the gear lived in overnight. It stopped corrosion from ever happening, and made gear overhauls ridiculously easy. I tried in with just plain water, but the daily dose of salt from the gear would turn the soak tub into a mild saltwater bath. Just the right amount of vinegar meant the rinse tub (which was emptied and refilled with fresh water with a dash of vinegar once or twice a week) would start slightly vinegary, and end slightly salty without ever being too much of anything.
And the gear always looked 'brand new' without the surface corrosion dulling the surfaces. It is probably not ideal for soft plastic bits, but the plastic and rubber bits are always much cheaper to replace than a dechromed piece of brass, replacement of which is which is usually expensive enough to make gear disposable rather than fixable.
