The vacuum barrel pneumatic speargun is a special case of the dry barrel speargun which dates back to the 1940's, in fact the first pneumatic speargun required a dry barrel to operate properly. A vacuum barrel is therefore a consequence of muzzle loading the dry barrel speargun when the diver is in the water, unless the diver has a way of tipping the water out of the inner barrel or displacing it with air after the gun is cocked. Neither of these alternatives are easy with a long gun, displacing water would require an external air pump or a controlled air bleed from the pressure reservoir to empty the inner barrel of water. The dynamically sealed or vacuum barrel arrangement is the simplest way to keep water out, but it has the disadvantage of the vacuum being lost and water sucked in with no indication to the operator that this has occurred. This problem has never been solved and as a consequence the vacuum barrel was regarded as a source of unreliability, so production pneumatic spearguns stuck to the familiar wet barrel arrangement.
It occurs to me that an indicator could be built into the vacuum barrel muzzle, but it requires a clear window (maybe more than one) that will not be blown out if the barrel has indeed filled with water. An air filled transparent object will have a silvery appearance underwater due to different reflections at the air to water interfaces with the object. Some years ago automotive batteries, which are lead acid based, had a "magic eye" (basically a Perspex rod) in each cell to indicate when the water level in the battery had dropped below a certain level, so this type of "time-to-top-up-with-water" indicator may be of use in a vacuum barrel speargun. If the "eye" or window is dark then the muzzle and barrel have taken in water as water behind the window will not reflect light, if it is highly reflective then the interior is dry. Tipping the gun upwards will send water in the inner barrel to the rear of the gun, so if the "eye" remains dark then your inner barrel is well and truly full of water, so be prepared for a weaker shot.
However this "magic eye" window is arranged it needs to be viewed from the side rather than the front end of the gun for obvious safety reasons! Possibly the transparent window can be installed in one of what were the muzzle relief port holes as in my view all muzzles should have some ports as emergency blow-off valves, otherwise your gun may be taking water into the pressure reservoir without your knowledge when you shoot with a flooded vacuum barrel arrangement. Although the pneumatic gun has oil sloshing around inside it, you cannot assume that it will protect everything inside the gun. During storage the oil and water will separate out (if there is enough of it) and where the water layer sits will determine where corrosion will be taking place. If you store the gun with muzzle pointed downwards then the water will be around the piston (oil floats on water) so probably no harm done with a plastic piston, but it may be a different story with the inner barrel wall, especially if the water inside the gun is seawater.
It occurs to me that an indicator could be built into the vacuum barrel muzzle, but it requires a clear window (maybe more than one) that will not be blown out if the barrel has indeed filled with water. An air filled transparent object will have a silvery appearance underwater due to different reflections at the air to water interfaces with the object. Some years ago automotive batteries, which are lead acid based, had a "magic eye" (basically a Perspex rod) in each cell to indicate when the water level in the battery had dropped below a certain level, so this type of "time-to-top-up-with-water" indicator may be of use in a vacuum barrel speargun. If the "eye" or window is dark then the muzzle and barrel have taken in water as water behind the window will not reflect light, if it is highly reflective then the interior is dry. Tipping the gun upwards will send water in the inner barrel to the rear of the gun, so if the "eye" remains dark then your inner barrel is well and truly full of water, so be prepared for a weaker shot.
However this "magic eye" window is arranged it needs to be viewed from the side rather than the front end of the gun for obvious safety reasons! Possibly the transparent window can be installed in one of what were the muzzle relief port holes as in my view all muzzles should have some ports as emergency blow-off valves, otherwise your gun may be taking water into the pressure reservoir without your knowledge when you shoot with a flooded vacuum barrel arrangement. Although the pneumatic gun has oil sloshing around inside it, you cannot assume that it will protect everything inside the gun. During storage the oil and water will separate out (if there is enough of it) and where the water layer sits will determine where corrosion will be taking place. If you store the gun with muzzle pointed downwards then the water will be around the piston (oil floats on water) so probably no harm done with a plastic piston, but it may be a different story with the inner barrel wall, especially if the water inside the gun is seawater.