Hello. I haven't done this yet but I think I will do it on my cruise in June. On another site (trip advisor), there are plenty of reviews and one from today. Here is the frist line from today's review I just read.
"The Hilo Hoppa On Hoppa Off bus is based very close to the cruise terminal. We had pre-booked our Hop On Hop Off experience, however it’s also easily available to book on the day!"
I haven't booked it yet. I hope this helps.