score:3
It is certainly uncommon enough that I have never seen this happen. Specifically, since you are asking about Balkan countries and you have tagged your post with Montenegro, I can say that this did not happen even once last month when I was traveling through 3 Balkan countries. For water taxis, we were quoted the price ahead of time but did not need to pay ahead.
We did take a small group boat which appeared to have a regular schedule. In that case, we bought a ticket a few meters in front of the dock and handed that over on the boat as proof of payment. This is not what I would call a water taxi since it is not scheduled on demand.