What does "I did but half fancy" mean?
"Fancy" means to like it - "but half fancy" means to not like it much at all. So he is saying that he did not admit to himself that he did not want to commit to the voyage before seeing the Captain.
It means he wasn't very keen on doing it. He wasn't too enthusiastic about committing himself to such a long journey.
As was mentioned by Oldcat, "half fancy" means to not be excited about traveling. Replace half with hardly and it makes a little more (modern) sense.