Comment on Contact - Barracuda Sub Fighter by Shimmering-Sword
Cool fighter Shimmy, guess you don't need propellers or jets if you have antigravity drives that operate independent of your surrounding fluid environment. 
But rail-guns underwater? That sounds like a bad idea, does the craft use its rail-guns only above water and the dart rifles and torpedoes when submerged?
But rail-guns underwater? That sounds like a bad idea, does the craft use its rail-guns only above water and the dart rifles and torpedoes when submerged?
What's wrong with them under water? Everything being relative, when the best we have right now is a slow moving bubble delivery system called a torpedo.
I mention this above in my own comment, but a railgun would instantly vaporize the water it comes into contact with. If you look at the footage and pictures of the US Navy's railgun tests you will note fire around the weapon barrel when it is discharged. Railguns don't use explosive propellant, that is the AIR catching on fire from the friction against the projectile.
Not all air, there are materials that are shearing off the rails and projectile turning to plasma. It's important to not just think about throwing a modern railgun under water and firing it, we all know what would happen there. But instead what happens when a railgun is specifically designed to work submerged including the use of advanced (even alien) technology. Things make more sense that way.
I don't care how much tech you have, when something moving at the kind of speed achieved by a railgun hits the water the proverbial shit is going to hit the fan.
Well, a very fast moving railgun projectile will quickly lose its kinetic energy on impact with the surrounding water- making for some pretty spectacular bubbles, I'd bet. But I am more worried about the cannons shorting out from contact with seawater- the rails have to be in contact with the bullet, and the bullet must be in a barrel open to the outside, thus water will probably get in there, the rails are charged up with a high-voltage high-amp current during firing, and water conducts electricity... which will make a short-circuit at the very least, and water in the barrel will resist the firing of the projectile. The railgun would probably explode. If these guys have built water-proof railguns that fire supercavitating bullets, yeah, it probably could work, just like underwater firearms do. 
But I am more worried about the cannons shorting out from contact with seawater- the rails have to be in contact with the bullet, and the bullet must be in a barrel open to the outside, thus water will probably get in there, the rails are charged up with a high-voltage high-amp current during firing, and water conducts electricity... which will make a short-circuit at the very least, and water in the barrel will resist the firing of the projectile. The railgun would probably explode. If these guys have built water-proof railguns that fire supercavitating bullets, yeah, it probably could work, just like underwater firearms do. As for velocity, the dart guns are also firing projectiles into water, the railguns will deliver more power so they should work better. They're not looking for over the horizon performance, just "shoot the guy in front of you". As for water in the firing system, I trust that engineers and scientists who work with nano technology and bio engineering along alien salvage can handle the wiring of a gun and water pressure 
Well one would assume that a super advanced underwater rail gun would have super advanced underwater projectile to cut through the water as opposed to impacting with the water and shattering.