I went the other way around last summer,

the ferry was fully loaded. I saw some cars were waiting for no-show spots.

The ferry from Victoria to Olympia seemed full as well. It took quite a while for all the cars coming off the ferry as I was waiting to get on. I guess one pretty much have to book a spot ahead of time.

I booked the ferry a month ahead of time and was made to be there one hour before schedule.