I think the location of the prop is going to be more relevant to performance than the voltage. A more powerful 24v would be less effective than a 12v if it was closer to the surface.
I suggest you look at the Maxpower thrusters than can be deployed when wanted, and withdrawn into the hull
when underway. Getting the prop even a further foot underwater makes all the difference.
We had a 12 v Maxpower on. 46 foot mono and it worked well but better safe than sorry etc, if we could have opted for a 24v I would do so.