All of these are very cool, very good ideas I think. Probably the mooring is the best in the short term. But, if they want to add more maritime type features I think the boat ramp and the harbour might be cool.
I hope jorbtar agree...
I'll throw out one more idea. Someone mentioned living on boats. This idea appeals to me, esp. given the nature of the rivers on this map. If one storage slot of the knarr were to be occupied by a metal anchor with a metal chain, could that not also be a claiming device? Actually, that might be better than the mooring! No new models would need to be made - just icons to be put into slots.
It could act just like a pclaim - needing to be fed by curios, otherwise it would "rust" out; can only be place so close or so far from another anchored ship. The only difference is it would only apply to the ship (and the water it was on), and could never be sunk into the land. It would need to have the LP drained from it before it could be taken off the boat. It'd have to be something like 10 bronze, iron, or steel bars, and 30 heavy metal nuggets for the chain. Or somesuchthing.