Is it correct to say "first rate hobby"? Is it "first rank hobby" or anything?
"Diving became my first rate hobby, and it seemed like my 24 hours of days passed in the ocean."
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In this case unfortunately neither term works. I would say "Diving became my main hobby" or "primary hobby."
For the second half of the sentence, more idiomatically would be "and it seemed like I spent 24 hours a day in the ocean."
I completely agree with Michael Scott Cuthbert.
I just wanted to add that describing something as "first rate" doesn't fit well here, but it is an expression we use in a different way. It's used to mean that something is among the very best of its kind, so it would not mean the same thing you are saying, but it would make perfect sense to say that diving is a first rate hobby.