Getting on and off the boat could be tricky, as there's that gap between the boat and the dock, as well as some stairs to navigate. It was easier getting on and off this boat than the Monterey Bay Whale Watch out of Monterey. That one can involve a ladder as well as a hop depending on the tide, whereas Sea Goddess wasn't that harrowing.
Space on the boat could be an issue, but one could probably find a nice spot on the back of the boat that would give a good view from both sides. It's a fairly small boat.
I definitely agree that you should call to make sure.
Also in Moss Landing, the Elkhorn Slough Safari can, I believe, accommodate wheelchairs. You probably won't see any whales or dolphins, but it is a really fun trip in which you will see lots and lots of otters, plus birds, seals, sea lions, and other wildlife.