For the most part, you'll want analog servos. Digital has a lot of advantages, but in a combat ship, most of those benefits are overkill. Digital servos provide better torque and response time than analog servos, tend to have better guts than equivalent analog servos, and can be programmed for range of motion, deadband, speed, etc. with a digital servo programmer. On the other hand, digital servos also are much more expensive than analog servos, and in most cases, a cheap $10 standard analog servo will do the trick.
To answer your original question, about what servos to use in your Big Gun Maryland:
I use hitec servos, so that's the product numbers I recommend. You can use the equivalents in whatever brand of servos you prefer.
Rudder: HS-311 standard servo: dirt cheap, and good enough. Circuitboard can be skotchkoted and case drilled to drain water, in case of sink.
Firing: HS-311 standard servo: again, this servo is plenty good enough to push firing buttons.
Rotation: if you're using the Morgret Command and Control system (coming soon!) or other positional turret control, then the Strike Models standard-sized sail winch servos will work.
If you're NOT using positional turret control, then you can either use a pair of HS-1425CR continuous-rotation servos, or gut a pair of HS-311's and drive them with a VEX motor-controller circuitboard. The first option requires a Y-splitter and a servo reverser, while the other option requires a soldering iron and some skill with wiring. I have personally seen both systems used in combat ships, and can assure you that they work far better than old hacked-servo rotation systems.
Depression: if you can fit HS-311 standard servos, I highly recommend that you do. The HS-311 is sturdier and more powerful than the micro servo many people use for rotation, and for less than half the cost. If you cannot fit an HS-311, though, the HS-82MG is the most popular choice for depression servos.
You should also consider starting out without servo-controlled depression. Even guns without servo-controlled depression can be adjusted via one or more screws, so you can still dial your depression in to the correct level. Plus, without depression to worry about, you have less cables to tangle, less electronics to spend money on, and less controls for your brain to worry about. Depression may be useful to have, but there's a very good reason why many ships go without.
Keep in mind that, while I have used Hitec servo numbers to describe my recommendations, nearly all servo companies make equivalents. For example, Futaba's standard servo is the S-3003, and Strike Models carries another fine standard servo, the VS-2. Any one of those will work fine for rudder or firing. Getting the right type of servo is far more important than getting the exact same brand as me. If you find a deal on a radio with servos, or a friend offers you some servos, do not hesitate to use those instead.
Lastly, try to pick one servo spline type and stick with it. I have seen a number of skippers driven crazy trying to replace a glitchy rudder servo because the spare servo had the wrong type of spline. There's a lot to be said for standardization.