Here is an article which was written in 2012.
The problem is that most pneumatic spearguns are designed for a pressure differential which is always in the same direction and hence the seals only have to operate in one direction. An "O" ring in itself has no directional preference, but the seat that it sits on may not seal equally well with the pressure differential going in either direction or during the transition from one side to the other. Certainly bi-directional sealing can be achieved and is used in many industrial hydraulic and pneumatic devices, but that will not have been necessary on the standard pneumatic speargun piston, particularly the ones that rely on the rear conical rubber or cup type seal and its flaring action under pressure which causes it to seal even better in less than perfect inner barrel bores. Those seals are also used in some hand pumps, one way they seal and push air, the other way they collapse their skirt and allow air to pass by thus causing the pump to breathe ambient air for the next pumping stroke. Water will equally well push past these seals by collapsing the rubber skirt on the seal if the pressure differential is reversed and that is a real possibility in a "sealed" vacuum barrel gun which has unintentionally flooded with water. What works in theory is not always realized in practice and I think this is why pneumatic speargun manufacturers have stayed away from designs where the seals may be overwhelmed and water can then penetrate the pressurized chambers inside the gun. Saltwater is very corrosive if it can get inside a pneumatic gun and it will then destroy smooth surfaces over a period of non-use where it can work on the same internal spot without being disturbed by any movement of the gun. Freshwater poses less of a problem and I think that this explains why hydropneumatic spearguns have persisted in places like Russia and the Ukraine and is also the reason that pneumovacuum spearguns have appeared there. Water on its own can cause corrosion, but salt in the water improves its conductivity and that allows a much faster reaction or galvanic corrosion to occur. Even 316 stainless steel, the most corrosion resistant of stainless steels, can have pits burned in it by saltwater over a period of time, so stainless steel inner barrels are not immune to corrosion.
I suspect that pneumatic speargun manufacturers, being realists, anticipate that their guns may receive little or no maintenance and hence have opted for designs which can tolerate a degree of neglect. If you are prepared to strip a gun down regularly and don't mind pumping it up on a frequent basis then more adventurous designs can be tolerated, but most gun users just want a gun that performs with little attention to maintenance and lasts for a respectable period of time before they need to replace it. Like light bulbs, manufacturers have some interest in spearguns not lasting forever and the introduction of the plastic piston has probably helped in this respect!