I ran out of boats in Silvar.
The first one I took straight to that area where I had a quest to kill someone in order to unlock Dispel Barrier.
The second one I took down the rapids, since I had 2 quests in one of the dungeons over there. I did not have a grey pass on me at the time though, so I couldn't complete that part.
The third one I took down the rapids again, but further west, since the first trip down the rapids didn't complete the quest for me where I had to map the rapids (which unlocks a trainer in one of the cities).
I now have the grey pass, so I need a boat to go back to that dungeon from earlier. I also need to explore some places up north past Fort Duvno I think it is. So I need 2 more boats.
I tried buying a 4th one, the npc takes my 400 gold, but no boat is at the dock. Is there something I am missing? Another source of boats somewhere or someway to get the other ones back up the rapids?