The "South" side of the island officially starts at the last stoplights in town, right next to the major cruise ship piers. It's after this light that the road splits into two. To the right, you'll find a one-directional frontage road that runs directly along the ocean, all the way to Punta Sur, the southern tip of the island.
The main road also continues to this southern point, but is two-lanes, bi-directional, and has a much higher speed limit. It's definitely a more efficient way to take a trip around the island, but certainly not as picturesque!
This part of the island is a bit different than the north part in that it's less populated, has a much longer shoreline, and has a number of sandy beaches. There are very few condos developments and no tall 10+ story condo buildings like you find on the north side. But there are plenty of villas, so if you're coming with a larger group there are several options for rentals that are directly on the ocean and/or beach.
If you're coming to dive, this is also a great part of the island to stay on as the two main diving marianas are located just after the aforementioned last set of lights heading out of town and the actual marine part where you'll be diving starts south of town as well. Several of the condo developments have docks for boat pickup and a few of the larger homes do as well. And unlike the north side, where dive boats generally won't pick you up since they're all heading south to dive, most dive operators will be happy to pick you up where you're staying on the south side as it's on their way to and from the dives sites for that day.