If you haven't already picked a dive op, while you are in Puerto Aventuras, may I recommend Dive Aventuras?
They will pick you up at your resort, and have a great combination of ocean and cenote dives available. We've been diving with them for six or seven years now, sometimes two or three times a year, and they are very professional. Our first cenote dives were with DA.
Thanks for the advice, Jim. I had intended to dive with Dive Aventuras while at the Spa Palace, because of recommendations such as yours. I'm still trying to figure out who to go out with while at the Valentin. We may just bag out on it that week, because as best I can tell, there aren't any operators who will pick you up, and one operator I contacted said it's going to cost $45 round trip to get to downtown Playa. At that point, doesn't quite seem worth it. But I'm still open to suggestions.