[Zeltino]

I think the parked boat and non-moving invincible, moving destroyable idea would get out of hand. Why not just set up some place - maybe another small island, where people not in kingdoms AND in kingdoms can build "house" boats and leave them in that area. These boats will not be able to leave that water. If they do, they are sinkable. If not, they are invincible.

This way, you aren't using invincible "house" boats to block enemies in battles or such stupid things as blocking paths so people can't move.

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For the harbour idea. Yes, the ticket idea I believe is the way to go - like I said earlier. You can possibly choose to pay for how long you want to keep your boat in, and depending on how long is whatever the price. You get a ticket saying you put a boat there, maximum 2-3 tickets. When you want to pull it out of the harbour, you can. If you leave it in the harbour longer than the specified time, maybe it is destroyed or is put outside the harbour for everyone else to use? That way, you could have different people getting boats other than the people who put them in there, without other random people just walking into the harbour and taking out boats that aren't theirs (as in, the tickets can be tradeable/droppable).