Wouldn't you usually want to keep the two batteries separate, and only use the House for cranking when absolutely necessary? Otherwise you'll be using them both for cranking every time, which means they'll both need to be charged from the charging system equally, rather than giving the Start battery preference. I'd have thought there could also be a risk of the relay sticking, thus causing both batteries to discharge rather than just the House.
I guess what I'm saying is that if it was a good idea, why don't boats designed with 2+ batteries have relay switches already fitted as standard?