It takes diamonds to recall a boat. Have Vouchers to send empty boats away.
Edit: Players got me thinking and I came up with this. The vouchers are not a mystery, it shows you the color of voucher you will get for filling the boat orders. Example you have a boat that has a green voucher. You don’t need a green voucher. So you don’t fill it but it takes a green voucher to send it away. What ever the color of voucher the boat has, it takes that color to send it away.
Or, maybe when you get to, let's say level 50. Then it would cost a voucher to send the boat away.

