Here's a possible corner case for you - if your ship's hold was full or almost full and you wanted to get a bunch of wood, you could spend as many fish as you wanted for extra 'moves' around the board any which way you wanted to clear space for the wood. Maybe I'm wrong, but I don't think voluntary dumping of fish or wood is otherwise allowed.
I only have access to the Prototype Rules from the Kickstarter at the moment, but page 19 shows the ship board and says:
"Fish and wood go in the cargo hold area. A player cannot have more than 10 fish/ wood at any time. A player can dump old cargo to make room for new cargo."
So feel free to get rid of those extra fish if it suits your plans!