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No, you only need a space in a boat to keep a fish card. To use a wish you don't keep a fish card, but you "throw it back", so don't need a space.
(See: Putting a fish card in a boat and Throwing a fish card back)
BTW You can never catch a fish and sell it immediately (as you mention in example #1. A double fish card is no wish, but you can only catch it and sell it in a later turn. (It's worth one fish when you sell it as an action. It's worth 2 fishes when sold by a wish).
I want to make sure I get this right. If this right?
Turn #1 - I catch a double-catfish.
Turn #2 - I use a wish from some fish in the ocean to sell all of my catfish. Since I used a wish to sell it I get 2 catfish.
Is that correct?
It makes sense...now I get it...the rules confused me.