I like the charter a boat for a week idea. Go somewhere awesome like the virgin islands for vacation. She'll have a great time and will associate living aboard a boat with vacation, making her much more open to the idea.
FWIW, this is exactly what got us started. A week charter in the Virgins, 6 months later we bought a boat and a few years after that sold the house. (Would've sold sooner but it took that long to find new jobs within commuting distance of the shore.) LOVE living aboard, we've been at it 7 years and no going back.
There's a difference btwn living aboard at a marina, and living aboard cruising. It's not completely clear from your post which you have in mind. Perhaps more conversation can really point out what the showstopper is - dislike of the itinerant lifestyle (wanting the kids to be in the same school, stable friends, etc?) or space contraints of a small boat, or fear of water? Life in a marina can be like a waterfront condo in a friendly community - ours has a park, clubhouse, swimming pool etc. I have to admit I'd be hesitant to move into a fixer-upper with 4 small kids - maybe a newer boat with a few more amenities would be more palateable?
PS to CD - nice post!