Ugh, stupid mouse closed the window and I had to retype this (that issue
isn't, for once, Vista's fault)
I don't plan on getting a PS3 for a while for pretty much the same reason, so I too watched the cutscenes from MGS4 on Youtube.
Outer Haven is just an alternate design of Arsenal, probably one of the rejected concept designs for Arsenal that Liquid Ocelot got a hold of and used his uber-income (as the secret leader of the largest PMCs) to get it built.
In combat it'd be just like Arsenal really. You can take the rail gun out of ship-to-ship equations, you'd only use it against entire fleets, since it launches nukes, and they'd have to be at quite a distance.
The thing seemed to dwarf the WW2 Battleship that Snake had, so that means it'd be pretty much the largest warship ever. Sheer size means it'd last longer, especially since I'm betting its got some very tough armour, considering the precious cargo it was built to protect (the AI called GW).
Being submersible is always an advantage, especially since it can launch it's main defenders (Rays) while submerged. That pretty much obsoletes aircraft carriers right there. It's got it's own missiles as well, though it's shown to have to surface to use those (that's actually just silly design-wise, we're perfectly capable of launching missiles while submerged).
It must have torpedoes too, really it would have used them in that fight at the end of MGS4. Outer Haven couldn't move to face the Battleship since it had to stay in position to fire the rail gun, but I'm fairly sure that modern torpedoes can be fired at a target that isn't within the intial firing arc of the torpedo launchers (see The Hunt for Red October, those torpedoes went all over the place! I realize it's a movie and so not necessarily fact, but it still makes sense to me).
In summary, it was clear that Outer Haven had the Battleship totally tanked. Though it was still awesome when a Ray got hit by one of the main turrets at point-blank range