Jen, you have 2 options when boarding in YVR:
1-- Get to the port early-- around 1030am
2-- Get to the port late-- around 2pm
My advise would be get there early. After checking in you have to go upstairs to be cleared by US Customs before boarding. Customs doesn't come on duty until 12noon, however the upstairs waiting area is very small and once filled they will keep everyone in the lower level until there's room to move them upstairs. Also, once upstairs you are given a number to be called for US Customs.. there are only about 3 or 4 Customs Officers on duty and it can be a slow process.
Last time we sailed from YVR, we got to the pier at 1030am, checked in, went upstairs got numbers for groups 1 and 2. RCI sets up drinks and cookies in the upper level and once Customs came on duty we were on the ship in minutes.
We observed LONG lines of people trying to get onboard all afternoon and were happy we did what we did...