When you hop in a boat holding an item and then place down that item in the boat when you switch to another item and try and place it down it will place the old item you were holding in your hand you are still holding the old item but your hot bar shows the new item. If you are holding nothing ot won't let you place anything. I tried it with other types of boats and still the same problem. 

I was in the new mangrove biom if that helps