If you do a hearth, make it cantilevered so it has that "contemporary" feel to it. No one will sit on it because the fireplace will be so daunting people will want to look at it from a distance. I don't think I would do a mantel, because with the size and the boat, it might be overkill. So, I'm not sure you need either. Think about "volume" and size and whether the boat will be too much for the wall given the stone and size? Here's a crazy idea, could the boat be incorporated into a hearth? I just have the feeling that the boat up high is going to create a big imbalance. If that is the case, then you definitely will need the hearth for balancing off the boat. Any photos you can post of this wall of fireplace?

Here is large fireplace with cantilevered hearth. Notice no mantel and nothing over it either. While the size of your "fireplace" is ginormous, I don't know the size of the firebox, but this one has always appealed to me. Perhaps it could be inspirational. Notice the way the stone is stacked and then the vertical stripe of smaller, but same color stones/tiles are laid top to bottom even under the cantilevered. Somewhat had great vision on this one! No mantel.

Another contemporary approach with the cantilevered hearth. Maybe this will inspire some thoughts for your house!

Thanks for that input! We've decided no hearth and no mantle after all. The fireplace is already so big we didn't want to make it up to even bigger!