Interesting idea, I'll be curious to hear some of the offered up solutions. Though I'm curious why you're thinking that you couldn't keep your car at a marina where you're staying? Care to extrapolate, please?
If that doesn't work, then there's the option of finding somewhere else to park it. And I know that when I lived aboard, while on a mooring
in San Diego
, parking my (older, semi beat up) car, nearby wasn't a problem.
Plus, there's probably the option of finding somewhere near your boat to park it, even if you have to move it a short distance every few days.
Be said parking areas within walking distance, or if it requires you to get to it via a bike, or public transport.
The bike option's a pretty easy one, & has served me well for decades. Plus, it just works, especially if you have quick release hubs on the wheels, so that it's easy to stow the bike in the car, while you're driving.
Just "ugly up" the bike fairly well, & get a decent lock for it. That, & carry; a tire patch kit, pump, & small repair kit, when you're using it.
One other idea, is to try contacting Tom Neale, who's an author that raised his family
while cruising in the Caribbean
& E. Coast. - They migrated seasonally, so as to optimize their locale for temperature, & hurricanes, etc... plus sometimes, simply when they found a good spot to call home for a while.
And as I recall
it, they had some "interesting" strategies for "storing" their (cheap) car, when they were out of the country/area for decent periods of time.
I read about it in this book http://www.amazon.com/All-Same-Boat-...ords=tom+neale
And if he's tough to find, via the internet
, or his publisher, etc. Odds are Cruising World
magazine may have a POC for him, as he did a lot of articles for them as well. And may still do so.