I think you make some very good points, but you're neglecting selling
fees, carrying costs before and after your
cruise, and one or two "gotcha" upgrades or
repairs that blow your economy of scale.
If you can manage
purchasing the
boat the day you're leaving on your trip, keep the
money put into it to a minimum, and sell it the day you get back, and do it without using a
broker, I believe your strategy might
work, though you might have to discount the
boat rather substantially in order to sell it in one day.
Pay $5K in slip
fees at any point during your boat
ownership, and that
money is gone for sure. Most of the money you put into personalizing the boat and making it safe and comfortable for you is going to be gone also.
I do think cruising can be an efficient means of travel with a lot of planning and a little luck, but I also think it's very easy to overlook a lot of the costs. I also think the more typical
experience may be to buy a boat at 50 or 100K, put 35K into it, and sell it for a little less than what it was purchased for.
I think in theory it's very easy to overlook some of the costs when doing this calculation.