Taking a sail or lantern etc. from a boat's equippable slots and holding it with your cursor, you can't drop the item. It's stuck to your cursor until you put it briefly into your regular inventory. This happens both when accessing the slots directly while standing on the boat and when accessing the boat's inventory through the Inspect option.
Steps to Reproduce
Get a row boat and a sail. Try to take the sail out of the boat's inventory and drop it.
Get a row boat and a sail. Try to take the sail out of the boat's inventory and drop it.