The problem with fitting brushless into subs is that most of the motors on the market are aimed at aeroplanes not marine use, and they tend to be wound for higher RPM, especially once you start looking at smaller motors.
That's not a problem if you are going for speed or have boats with very small props, but for a scale modern sub, it tends to be an issue, as they have larger props and require lower RPM.
The kilo isn't as bad as some, as it tends to have a smaller prop than many of its contemporaries.
If you're looking to save space, one of these geared down 2:1 is all you need. That will give you a shaft speed of about 7kRPM, which i think is about right for this model.
You can use o-ring belts for a model this size, silent, simple and cheap.
http://www.hobbyking.com/hobbyking/store/__6312__hexTronik_5gram_Brushless_Outrunner_2000kv.htmlDoubtful the boat can handle any more than about half a dozen watts without inducing handling issues due to torque induced roll from the prop. The motor above is rated at about 15 watts and is about the size of a five penny piece. You could with care, pretty much sink this into one of the end caps, so it almost became invisible.