I am a fanatic about good servos.
In the olden days (prior to 2000), an average standard size servo output about 40 oz/in of torque. Due to that, i personally feel any servo that produces more than that amount of torque will work fine in a submarine. even with a water tight seal, you will not be imposing more strain than what an airplane flying through the air at 100mph would be exerting against a servo.
having said all that... here is my two cents on what i use and why
I do not normally use standard size servos, since space is at a premium inside a wtc.
I refuse to buy the cheap $3 servos for my subs.
my go to servo for wtc's with room is the hitec hs225mg
my go to servo for wtc's with constrained space is the hitec hs82mg
the 225 is a mini servo, the 82 is a micro servo. both produce more torque than a classic standard size servo.
as you may have noticed, both those servos are metal gear servos. there is no use using a "high torque" servo if all you will be doing with that excess torque is stripping servo gears.
you might also notice neither of those is digital. submarines will not need the precision afforded by a digital servo, nor does the extra drain a digital servo can place on an electrical system help in a submarine. on average, an analog servo is also going to be cheaper than a digital (although the metal gear option will increase cost)