Not many bomber boats with propulsors/pumpjets. Vanguard is one, and a Vanguard at that scale isn't that long, so as you're a Russian sub man, I'm thinking this must be a Borei class?
Pumpjets tend to develop more thrust at lower RPM than a free running prop, so dependent upon pitch of the stator you may want to go lower in RPM still.
For instance the Prop Shop propulsor I have for my Vanguard needs only about 2000RPM to generate plenty of thrust, as the props pitch is very high. To push it beyond 2000RPM requires serious poke, I had a large Buhler motor drawing 60 watts to push it to 2500RPM.
Direct drive versus gearing? This is a much debated topic, with direct drive, you will usually need a physically larger motor to obtain the required torque, and the armature of the motor will have more windings (electrical gearing), which makes for more electrical resistance and thus lower efficiency.
With gearing, efficiency is lost through friction in the components. Belt or chain drive is the most efficient, about 95% transfer efficiency. Gears are less efficient, 90% at best, although can be as low as 75%. Most of this isn't worth worrying about for small models like ours, just pick a combination that works best for you.
For Akula the Engel 12 volt 500 size motor works great direct drive. That's a high wind, low speed 500 motor that spins at about 5700 RPM on 12 volts. http://www.engel-modellbau.eu/catalog/p ... g78po4lab5
Engel use this motor in Lafayette and 212 kits, and it powers both very well. The Akula is a much fatter hull than the Lafayette, so expect slightly less speed.
The 720BB (not 700BB, that's a different animal) is a low revving high torque motor that puts out about 36 watts of power, and runs at about 4100RPM on 12 volts. It can swing up to 60mm props direct drive, perhaps larger dependent on pitch, beyond that you need to gear it down. If it's well matched to the prop, it should draw about 36 watts under full load.