Both sentences are good. I'm not sure what "sea fishing" means. Two possibilities come to mind. It could mean that you went out in a boat and fished on the open ocean. The other possibility is that you stood on the shore and fished from there. You might want to use a more specific expression if the difference matters to you.

Your initial phrase is unnatural in American English. We don’t say ‘sea fishing’. We would say ‘salt water fishing’, or be more specific with the terms ‘deep sea fishing’ or ‘bay fishing’, with the difference being the size of the fishing vessel required. Even correcting that, alternative 1 is incorrect and alternative 2 has a different meaning. You could use ‘to do’ in the following way, however: I went on a salt water fishing excursion in New Zealand. (Better) I did a salt water fishing excursion in New Zealand. (Worse, but something native speakers would say) I went on some deep sea fishing trips in New Zealand. (Better) I did some deep sea fishing trips in New Zealand. (Not quite as good but OK) ‘I was fishing’ and ‘I went fishing’ are both grammatically possible but don’t have the same meaning.

Hi Kevin, the most natural would be your original sentence “I went fishing in New Zealand”. In this context, ‘I went’ is more natural than ‘I did’. Additional options: * When I was in New Zealand, I went fishing. * When I was in New Zealand, I went fishing at sea. * I went fishing at (the) sea when I was in New Zealand. * I went fishing when I was in New Zealand.

Number 2 is fine. It would be more common to say fishing, rather than sea fishing