The bow thruster in my Neddy is powered via its own speed controller, of course, but still working off the same battery as everything else - except... the mast lights, which are powered by a separate 9v battery. Reason? Just in case of power failure during a night sail. At least I'd still be able to see her and guide one of the other club members' boats in to attempt a rescue.
Neddys turn pretty much on a sixpence anyway, but the thruster gives that extra bite when spinning on the spot, as well as correcting the tug's direction when towing something large.