This past August we had a return flight of 1:00 out of Vancouver. We did the express walk-off about 8:30 a.m. and caught a cab really easily. There just wasn't a line that early. We were to the airport way early. The customs line wasn't long and moved pretty quickly. I think you'll be just fine with a 12:30 return flight.
Another year we we waited until about 9:30 or 10:00 to leave and then there's a huge line for cabs. We still were very early for our 2:00 p.m. flight.
Have a great cruise