Ooh! A mini-DO at Taco King! I'm game!
As far as rentals squeezing on ferries: the smallest rental car I've seen is Enterprise's Chevy Aveo (which I wouldn't wish on anybody), but if you reserve one with them, you're likely to get a free upgrade to an extended-bed Silverado, so I wouldn't count on that. Most of the other companies up here have gone mostly Chrysler (they're the only ones doing guaranteed buybacks now), so the smallest available would be the fairly large Dodge Caliber, although you might get a Chevy Cobalt or a Toyota Corolla. Best advice would be to call the agency's location directly (use www.acsyellowpages.com--that's our local phone book) and ask what their economy or compact vehicles are. However, summer is the time of free upgrades (yes, even to minivans, like it or not) because companies are basically completely sold out from June 15 to August 15. You get what they have.
Double check with the rental company to make sure they allow vehicles on ferries--I know at least two don't. You can try to get a special exemption from a manager or you can simply try to sneak it on the ferry without them knowing (they probably won't know, anyway), although then you run the risk of having the ferry people not let you on (I have no idea if they enforce any such rules).
(YAY! Let it be known for the record that this post is the one which finally qualifies me for OMNI!!!)