Second Lambada's motion. We've taken the ferry from PR to DR. So far I believe it's been around 10 times in the last few years and though I prefer flying since you don't waste so much time, it is the only way to get the vehicle across.
The ferry takes a day going and a day coming. Basically you begin your day and show up at the dock at around noon and get all your papers, got through imigracion, aduana, police vehicle check and the rest. Ocassionally the enterprenuring public servant who will want a small "mordida" to make your day more pleasant, though this is becoming a rarity of sorts since a few have been put out of service.
They begin loadiing cars and SUV's at about 4:00Pm and finish pretty close to 7:00PM. The motorcycles are always the last which is a good point since they are almost always the first to get off.
From there it is up to the cabin to get all the grime and gunk off, get some food, get some drinks, catch the show, gamble some bucks away and hit the sack. Wakie wakie next morning to get off the blooming ship and do all the paperwork on the US side again. The line for customs, the line for imigration, the police check on the vehicle, the dogs to smell for drugs. This paperwork routine is what turns a good ship ride into a test of patience. But what the heck, if you know it's coming then you put on your best face, take an amiplin500mg. or a sejodio1000mg. and you make the best of it.
Once in PR you can travel everywhere as this small island of 35x100 has paved roads going everywhich way.
Enjoy your trip.