FLJ, fair comment. I haven't heard them all, of course. Alas, I couldn't find one of those engines, except possibly a Gray, since they are the preserve of the Americas, to which I cannot go and I have no means of recording them anyway, but it might well be an idea for you to do when you are attending shows or driving round the land, to take in a meeting of the Classic Motor Boat Association where Chris-Crafts do attend along with Albatrosses and Deltas, Healeys and various outboard powered boats. One big six is likely to sound like another.
Of course a lot of these kind of boats now have modern American V8s installed, and they called be recorded at any hot-rod show. I assume you run it as a loop, so a few seconds of steady running would do. But that watery burble can only come from one place.
And that is SUCH a feature of a woodie/speedboat that I think it is far from a gimmick. It would cause surprise and a smile and what could be better than that?
But i mean it...if you can come up with a realistic old big engine burble, I'd buy one! And you won't hear that very often from me<G>
Regards,
Vintagent