I couldn’t picture where the official cruise parking was either so I did some digging and found more info on the official cruise parking website:
https://www.cruiseparking.co.uk/ports/liverpool/
It seems you drop off your luggage at the port then drive to the car park at L1 8JU and then get a shuttle back to the port. The car park is located quite a bit further along than the likes of John Lewis Q Park and the Ibis Albert Dock, which are both walkable to the M&S Bank Arena. I’m more than comfortable with having booked the Ibis irrespective of the fact it was cheaper than the official cruise parking.