I think it depends somewhat on where you plan to mount the watermaker. The washdown through hull came from the factory in the ER, forward, port side, but I don't think there is much room for a watermaker there.
I've been considering mounting the membrane unit from our rainman in the lazerette. If I do it, I will add a through hull there.
I know you can run a hose, but it just seems to me that if you are starting from scratch, the shorter the hose run the better.