Hello Hitomi,

I understand what you are trying to say and that's great! Here are some suggestions.

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I'm very excited! (ok)

I'm going diving in Japan today.

I never doing it before. I'm very interested in.

I have never done it before! I'm very interested in it! OR I have never done this before and it's so interesting!

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Hope these help you!

Have a great time learning English (and also diving)!

Regards,

David