Xanana - yup, taxis ain't that cheap - if you plan on travelling out of town (e.g. to the beaches or San Gervasio) then a rental car will probably be cheaper.
Dave56 - actually I agree! San Gervasio is worth a visit if you are really interested in archaelogy, but otherwise it's not that exciting - especially if you've been to any of the bigger Mayan ruins on the mailand like Chitzen-Itza or Tulum.
To drive or not to drive?
I guess it comes down to individual confidence/experience, if you're even thinking twice about it then it's probably best not to drive.
There's little chance you'll end up in jail (unless you're drinking and driving or doing something equally irresponsible). More likely is that someone hits you, it's their fault, but you'll have to pay. Even if you have insurance you'll have to pay and claim the money back. If you can't pay you could end up in jail I guess, but this is unlikely - the police will be more interested in your money than in spending time on paperwork and taking you to jail.
In theory, all this could delay you and make you miss your ship, but I've never heard a case of this happening first hand...
My best advice would be: if in doubt - take a cab!