Last night I had the opportunity to ask a Glock LE sales manager about maritime spring cups. He told us that they were not intended to allow the pistol to be fired underwater, and that doing so would probably result in serious harm to the shooter. Think about it: lots of sudden high pressure close to you in water..sounds like a lithotripter to me...the thing they use to break up kidney stones. Not to mention what it would do to your head.
Instead, he said that the maritime spring cups were designed to allow water to drain from the pistol more quickly. In cases where the pistol is carried by people whose mission calls for them to wade (or swim....), this is a necessary feature. With water drained out of the pistol more quickly, it will operate safely and predictably. That can't be said for a Glock which has water in the firing pin channel.
He also said that the G19 is now the favored pistol of SOCOM...but that's another topic, I guess.