Do we know already everything about the "Great Galeass"? Arioch states at his website, that there is still one unknown promotion. Why is everybody asuming, that this promotion is not inheritable? Well, maybe it is not - but we don't know, so far!
The Great Galeass as Venecian's UU is perfecly fine. Historically, the Venecians "invented" this ship class as a powefull trade-ship to be protected against the increasing pirate mischief. If at all, it might be a little bit unlucky that the galeass is a generic ship in Civ5 already and available for all nations. But as we have to live with this, a "Great Galeass" works well enough.