Just thinking about water getting inside the gun's pressure chamber and turning the oil "milky". Water can get into the gun through the pumping process when charging the gun at the start of the season due to high air humidity, however the amount of water is very small and unlikely to be noticeable in the oil. If the pressure acting either side of the piston seal (or seals) equalizes then the mid-body "O" ring loses some of its sealing abilities by not being pressed hard against the wall of its seating groove and the rubber cone type tail seal can allow water to penetrate the pressurized air area as its flaring action against the inner barrel wall is temporarily inhibited. That will only happen when you have hydrobraking as the piston hits a volume of water inside the inner barrel which cannot escape and there is no air present to soften the blow by compressing instead. Hydrostatic pressure increases instantly as water is incompressible, however you would expect that this hydrostatic pressure should rapidly blow the sealed slider out of the gun before that pressure ramped up enough to bypass the piston seals and send water backwards into the gun. However without the "emergency blow-off" afforded by a sealed slider, muzzle systems such as the Tovarich and similar arrangements using a front tied spear may allow the hydrostatic pressure to spike just enough to allow water to penetrate the gun momentarily. So with these guns you need to keep virtually all water out of the inner barrel as there is nowhere for it to go until the spear tail exits the muzzle opening.
On spearguns using only hydrobraked muzzles there is a small flow path that allows water to escape the muzzle at a restricted rate, just enough to slow the piston down immediately before it hits the travel stop without building hydrostatic pressure in the muzzle to a level where it allows water to enter the gun's air spaces by overwhelming the piston seals. Tromic's sliding rubber cover, which seals off the converted muzzle's unused relief ports, acts as a hydrostatic pressure relief valve, so with the correct tension it should eliminate the problem of water entering the gun and contaminating the oil inside it.
