See the last line on "fares" page. $15 off season, $20 peak season.
https://www.ferries.ca/thecat/fares/
I took the
Cat years ago when that vessel made the now discontinued trip from Bar Harbor. A fast ferry, bring your dramamine!
Back then the trip from Portland was on the
Scotia Prince, a slow overnight ferry with cabins. A few years later a luxury ship, the
Nova Star, took over. It lasted only two seasons and now the Cat is doing the Portland run.