I was there about 9 years ago, but most of it hasn’t changed much.
There are only two boat companies that go there from the mainland. I took CATALINA EXPRESS to/from there out of Long Beach, CA. You can board the boat in your wheelchair and they will assist you. There are no tie-downs, but I was okay with that. I didn’t use the restroom since it was a short trip. You can’t get up to the higher level, so just get the basic fare.
You’ll arrive in the town of Avalon. I can’t speak about hotels, since I was only there for the day. I found the shops and restaurants to be accessible. I was able to use the sidewalks with ease. Some are a bit steep, so not the best for manual chairs.
Overall, Catalina is best if you’re going for a quick getaway with your partner/spouse/bf/gf.