It sound like you are planning to go on Adventure of the Seas. Wonderful ship, great itineraries, you will love it. This ship docks at the International Pier, so hotels in the Condado area would be convenient. We stayed at the Wyndham Condado Plaza, could see the cruise ships from the small balcony. Also have stayed at the Sheraton Old San Juan when it was a Wyndham, that is across from the pier Carnival uses, and right in Old San Juan for sightseeing. This year we are staying in the ISla Verde area, a Hampton Inn, haven't gone yet so can't comment.
Taxi from airport to Old San Juan Hotel was $18 for two, don't know pricing for more, but outside airport you go to a desk, tell them where you are going , they will give you a price and get you a cab.
I think boarding starts around noon, but not sure. We stayed at our hotel, they locked up the bags for us, until about 3, took a cab to the pier, and boarded with no lines. The ship doesn't sail until late, so there is plenty of time to get to the dock. Missed the welcome buffet though, that was a bummer, but was glad to miss the lines. Hope this helps.
there is a nice hotel called Normandie very clost to the port at around 50-70 per night. you will pay tons more at the Holiday Inn or Sheraton. Trust me.The ship arrives back in port at 8am. For all of the people to disembark, clean the ship and all else that must be done it does take time. You may usually start boarding in the early afternoon. Think of it as a day on the ship. Take the day to familiarize yourself with the ship.