Contact Belize Expeditions and ask about a tour of Lamanai. When I lived out at the lodge we did a number of ruins tours for elderly or handicapped persons using a chair sling and four bearers.
They will arrange pick-up, transportation by land and water and a ruins tour with lunch that will outdo most any cruise ship tour. www.bzexpeditions.com
(I designed and built the site, just to blow my own horn)