yes, it usually is. There will be taxi drivers waiting near the piers, and the islands are mostly fairly small so the things you want to see won't be far away. Haggle over the price if you want, but it will probably end up about half the price of the ship's excursions. You'll need to tip but you'd probably have to do that on an official excursion too. The driver might or might not give you as much information as a proper tour guide - if you want lots, you could try to buy a guidebook like this
http://www.amazon.co.uk/Frommers-Caribbean-Por ts-Call-Complete/dp/0470289716/ref=sr_1_4?ie=U TF8&s=books&qid=1226840902&sr=1-4
One thing to beware of is that if you're late back from a private excursion, unlike an official one, the ships say they won't wait for you (though they often do). So make sure you don't linger too long to get back on time.
Which excursion you go on depends what your route is - which OV are you on, 1 or 2?