When you play a Pirate Ship with no Coin tokens on your Pirate Ship mat, the prompt in the log area that wants you to decide between "attack mode" and "gain money mode" reads:
Choose one: + or attack.
The same sentence appears in the central information area.
I assume that because there are no Coin tokens on your Pirate Ship mat, the "0 coin" value that is about to be displayed gets removed because "0 coins is nothing".