Technically, it would depend on the outdrive. In the case of a Bravo III, it would be BOTH. LOL!!!!
For some reason, I thought that inboard and Vdrive Merc's are RH and Sterndrives were LH. I also seem to recall the standard outdrives being LH as well, with the 'counter-rotating drives being RH. I also thought that you handled the counter-rotation in the outdrive on a sterndrive, not a counter-rotating engine.
I don't think it matters on a single whether you have a RH or LH as long as the prop matches the rotation. Which is standard? I'm not 100%.