There are some misconceptions about Glocks and shooting them underwater.
You can shoot a glock underwater without modification, however – water does not compress, and the shock of repeated underwater shockwaves can do major damage to a person’s internal organs. Therefore, this is one of those things that, just because you CAN, does not mean you SHOULD.
But besides the impact to your body, the water does impact on the gun. There are several very cool you tube videos showing high speed camera footage of underwater gunfire. When shooting on dry land, the atmosphere does not impact on the gun the same way the water does. Since the water does not compress, and it flows slower than air, water can bind in the firing pin chamber and cause your Glock to fire as a single shot. To remedy this, Glock created maritime spring cups.
Spring cups are two half cylinder shaped stops that fit in the firing pin spring and cup the firing pin connecting the two so that the spring can cause the firing pin to function. Spring cups are called the “million dollar part” because if you break them or lose one the gun WILL NOT function.
Normal spring cups are solid plastic, maritime spring cups have two channels cut into them to allow water to flow past them and empty out of the firing pin chamber, so that it can move fast enough to fire the handgun.
Maritime cups don’t actually allow the gun to fire underwater; they just allow the gun to function more reliably when wet. While the gun CAN fire submerged, the idea is that when working in maritime environments like on rivers or swamps, if the gun is submerged you can pick it up and start firing without having to shake the water out or perform maintenance. In the Glock 18 courses they actually fire the full auto glock until it smokes, dump it in 5 gallon buckets of water to cool, and then return to firing by using the maritime cups.
Unfortunately enough people (knuckleheads everyone) have tried out their maritime cups in their bathrooms, toilet bowls, and above ground pools that Glock considers the spring cups to be restricted items. Only law enforcement agencies can buy factory maritime cups, and only on official letterhead signed by the agency head.
You can buy aftermarket cups, but you should know that they have less material to support the firing pin, so they are more prone to failure, they are not able to be seen without disassembling the slide, and if they break the gun is useless until repaired.
Spring cups are always replaced in a set of two, Glock only sells them in pairs, and factory cups are only a dollar. I do not see a need for using the maritime cups, but if you do – I am not mad at you, its your gun.
Just please be aware of the entire issue before you decide to tinker…