Greetings,
THREAD DRIFT ALERT!!!! Mr. R. "...an underwater pod to stop the prop sucking air ( ventilation)" Not if, as I suggested, an electric or hydraulic "motor" (much smaller physical dimensions and much less need for service access) was put in each pod in lieu of the diesel engines that are usually shoehorned in. The main, single engine could be amidships on the aft deck allowing 360* access for service purposes.
One would still enjoy the separate cabin areas, massive saloon and have all the advantages of a diesel cat without the contortions for performing simple maintenance at the aft most end of each hull whilst standing on your head and potentially soiling the bedsheets...