The SeaBus is accessible to wheelchair users. Both terminals have slopes down to the water to reach the boats, though the one at Lonsdale Quay is slightly less steep. In both stations tickets are purchased at the kiosk but there is usually an attendant nearby if you need assistance. Furthermore, both terminals have a couple of transitions in the walkways that create bumps but are clearly marked in yellow.
Not every door to the SeaBus is designated to be wheelchair friendly so look for the universal symbol in front of the door’s entrance. There’s a few. The ramp is a little steep but short and once inside the door you will immediately see the space for wheelchairs right near the door.