Here are the major differences:
In Revised, all subs fire in Opening fire, but their casualties are removed immediately only if there is no enemy destroyer.
In Pacific, only attacking subs get First Strike (firing before everything else), and only if there is no defending destroyer.
In Revised, subs may fire and then submerge.
In Pacific, subs may fire or submerge.
In Revised, subs submerge at the end of the combat round.
In Pacific, attacking subs submerge at the beginning of the combat round, while defending ones submerge during defender fire.
In Revised, the presence of an enemy destroyer keeps subs from submerging.
In Pacific, it doesn’t.
In Revised, subs can always be hit by air units.
In Pacific, subs can be hit by air units only if there is a destroyer friendly to the air units in the sea zone.
For differences between Revised and Anniversary, see this document. Guadalcanal’s system is so different as to make direct comparison pointless.