There is a great deal of misinformation about Glock’s out there. From the ridiculous like it being made out of porcelain to it being the ultimate underwater weapon system.
I saw a claim stating that the Glock is the only pistol able to fire underwater because of the reduced friction from the polymer frame. Laughable.
Many firearms will work underwater with a few conditions. First there can not be air trapped in the barrel as that can cause catastrophic failure. Water will slow the fall of hammers and the movement of parts, so weapons with weaker or less reliable ignition systems may fail. Hollow points and similar expanding ammunition may cause failures by trying to expand in the barrel when fired.
There have been various documented tests out there showing 1911s, Revolvers, and other firearms functioning fire under water. Most of the times you hear about firearms blowing up from water in the bore it is because the firearm is out above the water while water is still in the bore.
That is not to say that firing underwater isn’t dangerous. Shooting is dangerous to begin with. There is more that can go wrong under water, and since liquid is effective incompressible, the noise, shock, and blast from the shot carry far better than they do in air. That means that the blast will be far worse on your ears and rest of your body.
You can find videos of people firing rifles underwater. That gets a little more complex. Similar thing, it is air in the system or bore while underwater that can cause failures when the gun is completely immersed. Or water in the while when it is taken out of the water. There is plenty of discussion and information about weaknesses in the AR15 design for over the beach use. Funny thing is there is a story about a guy who completely immersed his AR15 and was having no issues shooting it underwater. It was only when he pulled it back out of the water that he blew it up.
But I am getting off topic. Point is, most guns will work under water, but it is not ideal.
At the request of some group or another, with a Florida police department usually getting the credit, Glock designed modified spring cups for maritime use. These are not necessary for using the Glock under water, they simply exist to aid in reliability if there is water in the striker channel.
Now there is a cost to them. A set of maritime spring cups would run you 10-20 dollars and about 5 minutes to install them. These maritime cups have less material, and may fail sooner or be more likely to fail than the standard spring cups. I haven’t ever heard of a set of maritime cups failing, but it is possible.
It can be a little hard to find factory Glock brand maritime spring cups. Plenty of unscrupulous dealers will buy aftermarket connectors, maritime spring cups, etc and claim that they are factory Glock parts. I had some “Glock 3.5 Lb connectors” that causes reliability issues. I later found out that one of the aftermarket companies (maybe Scherer?) had a run of defective connectors. When I later bought more Glock minus marked connectors, those looked different and had no issues. Unfortunately the dealer I used to trust years ago went out of business. Went or jail or something that. Maybe I shouldn’t have trusted them either.
I live in an environment with a good bit of water so I choose to run the maritime cups. I needed some more so I shopped around for a while and saw several reviews of people claiming that the ones they received were aftermarket. I finally found a dealer that had great reviews and bought from them. Unfortunately the ones I received just came in a unlabeled zip lock bag, so I question their origin.
Buyer beware, I suppose.