Maritime Spring Cups were designed to accommodate a request by a special operations military group who was concerned about the ability to fire their Glock pistols after they had been submerged underwater. The ports in the Maritime Spring Cups were engineered to allow water and debris to empty out of the firing pin channel, preventing malfunctions.
While this might seem beneficial, the ability for more foreign substances to come out also means that more foreign debris can come in.
The fact is, you can still shoot a Glock underwater even without Maritime Spring Cups, though we don't officially endorse or recommend it. Your goldfish and your toilet bowl will both appreciate your restraint.