What specific "snap back" are you referring to? There are generally 2 types...
1 - Metal on metal, with no seal - these have zero water resistance.
2 - Metal on metal, but an O-ring is present inside the case back, that provides a seal - these can be water resistant.
The third type is more recent, and uses a hard plastic seal - these are easily water resistant with a new seal.
As already noted, a lot depends on the condition of the sealing surfaces on the case back, case, and of course the case tube and crown - crystal also, but it not typically a surface that gets corrosion all that much.
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