As OP, I was interested to know when was 'the soonest' I could book a show. before my first VV Cruise. Six cruises later I discovered the answer was not unit I first boarded the ship.
I'm speaking from my experience at Terminal V in Miami only but I could never access the ship's wifi before I boarded. I tried almost everything including trying including trying from the 5th-floor parking garage a stone's throw away from the ship. My experience was always before 2 PM because, as a Sea Rover, I was always in my room by then.
I guess It is what it is 🙂