@25:
Of course there are expenses other than dockage, but there are also expenses other than the mortgage costs as well.
Maintenance costs can be largely avoided if you do the really routine stuff yourself, like oil changing, winterizing & reconditioning, bottom painting, etc. Example: My boat needed a rudder angle indicator and autopilot at the lower control station. I purchased the control head, built a housing that conformed to the existing console, and wired it up. Total cost was less than $450. Had I let the people who supplied the unit mount and wire it, I would have been looking at about $1500. In the interest of full disclosure, I have a degree in naval architecture and marine engineering.
For marinas, you might need look outside the traditional areas. For example, my marina is 17 miles south of Annapolis. My slip in Annapolis would be $10k to $12k for a marina that is not as commodious.
Safe havens from hurricanes are obviously an issue. However, I would point out that, unlike a house, a boat can be moved out of the path of a storm, or hauled up on shore.
Is living aboard a cheap way to live. It can be if you want to live a very denied life. However, for maintaining a reasonable standard of living, it will be generally less expensive than living ashore.
$70 to $90 per hour is a huge amount of money, and implies that you are using a lot of horsepower to move fast. My 390, a trawler, uses just 2 gallons per hour at 7 knots, plus about another quart if I'm running the generator. Total price today, less than $6 per hour underway. The engines of today will run 6000-8000 hours before they need major maintenance.
YMMV!