Steve:
We found a place relatively close to the airport when we did our Dominica trip thru SJU. Especially if you're getting in at 10pm the night before. You're not likely to want to do much but take a walk with the camera and/or grab a leisurely breakfast/early lunch before the flight to Dom anyway. There's a fair list of places along Avenida Ashford or Avenida Isla Verda that aren't that far - that time of night should only be a 10-15 min taxi ride. Getting something for dinner that night might be challenging outside of a hotel restaurant.
They aren't kidding about luggage restrictions into Dominica. Confirm with American as it may have changed (the airport has been working on a larger runway I believe) but when we went in 2008 it was 70 lbs total maximum in 2 checked bags, vs. the usual 2 checked bags with 50lb per bag limit. They didn't weigh my carryon and did let me get a Lowepro Vertex AW300 on the plane with most of my gear (Ike housing, DS50s, body and lenses, only the big 8" dome and tray/handle went in with the dive gear into the checked bag, carryon was about 33 lbs.)
If you haven't been to PR before either then schedule an extra decompression night between your DOM>SJU and SJU>AZ legs(I mean in terms of 'relaxing after the trip' not in terms of actual nitrogen decompression) on the island on the trip back and stay somewhere further out in old San Juan. Makes for a nice day of wandering around with a camera, although its gotten kind of seedy. There was a really cool place called the Gallery Inn that we stayed at that was really fun architecturally and in terms of all the sculpture and the little courtyard (more a 'B&B' than hotel), but more recent reviews have been kind of mixed about the actual service. The rooms in the big casino hotels are nice, but incredibly overpriced and very impersonal.
Hope it goes well. I'm giving serious thought to planning way ahead for late 2011/early2012's whale season. Besides, Izzy hasn't had a new reason to laugh at me for some time.