Does saltwater damage electronics?
Salt water is an excellent electrolyte, and will accelerate galvanic corrosion reaction between two different metals especially in oxygen rich environments (including dissolved oxygen in water). This may create metal oxide deposits on circuits. The longer the exposure time the more corrosion can occur.

If your device is on, that is, there are circuits with electricity flowing, clearly adding a salt water could also short circuit your device as well.

As to whether any of the above results in irreversible damage is hard to say with certainty as some devices have been dried out and appear to function at least following exposure to common tap water.