Talking about the trip I did a few years ago:
I have been there and it was amazing. I had not seen a frozen sea until that time!
Did I use past perfect correctly or should I just say "I did not see .."? Also is the "until that time" ok?
The verb forms are fine, but, depending on context, until then might be more usual than until that time.
The usage is correct, but I would consider trying
I had never seen a frozen sea before!