Are you staying in Playa? It definitely would not be worth the drive to Akumal to dive, if so. It would probably take about the same time, maybe a little shorter, than taking the ferry over to Cozumel. The ferry would be more expensive though but you get my drift?(no pun intended!)hehe, bad joke. You could try the diving north of Playa if Playa is your home base for the trip. Puerto Morelos is supposed to have a nice reef a short boat ride from shore, though this is word of mouth as i've never been there myself. As for the southern shore, in my opinion, unless you are staying in Tulum or the Akumal area, it's not worth the travel time to dive OW here. I think you'd have to drive way down to the Costa Maya(Mahahual area) before the diving becomes worth it, but this is a 3hr drive or so from Playa so a multi-day stay in this area would be necessary. If so, try to get out to the Banco Chinchorro. It's a 1.5hr boat ride from Mahahual, but it's virgin reef with nary another dive group in site. By the way, i'm predicting Mahahual will be the next Playa del Carmen, though far off, but the Mexican gov. is supposedly planning to market and develop this area the way it did Cancun and the Riviera Maya a couple decades ago. I think they're even planning another int'l airport. If you want to dive this area before all the development, i think now is the time but on the other hand if you like the bustle of developed areas such as Playa, then waiting awhile is best.
Sorry, i can never keep things short but i'd say if you are set on staying in Playa, focus at least 60-70% of your diving on cenotes and 30-40% on OW. So if 5/2-tanks are your plan, then 6-7 cenote dives and 3-4 OW dives would be good.