Alot of manufacturers in the eighties went the cheap and simplistic route using Brush motors , For example ………… Phantoms or Sprints. If size is not a problem use 220v and magnetic coupling.
The real big problem Phantoms had were water ingress through the 1/4 orifice ( Shaft ) due to fishing line getting trapped and bending causing friction on the 108 orings , 117 orings Inner and outer nylon bearings and the oil leaking out , This then caused the Water to flood into the ROV completely as there were no water blocks in the tubes , well …… apart from the lateral thruster which I and my brother enclosed in a bit of shit pipe and kept serperate !
The sprints problem with the thrusters was dust build up on the armature causing noise on the video or Thruster heating up and failure. ( But as long as the Thrusters were "blown-out" periodically as in maintenance there were no problems . 🙂
You say you have little power to play with , What power are we talking about ?