The drive from San Juan to Guanica takes approximately 2.5 hours. You will be on Highway 52 all the way through Ponce. This road goes through the mountains but it is very smooth. Watch your speed. In PR, the cops ticket more aggressively. Make sure your order the Toll Option with your rental car. You will be able to go through the toll lanes and will be billed later by your rental car company. Don't worry if the red lights flash when you go through the express lane. The toll administration will know it's a rental car. At the end of Highway 52, you will go west on Route 2 towards Mayaguez and get off at Route 116 towards Guanica/Parguera. Just follow the signals. Personally, I prefer La Parguera over Guanica when it comes to restaurants and life but that's just me. I have dove with San Juan divers in the past and they are a good operation and, yes, they are closer to the wall.
Just returned earlier in Jan from diving in Puerto Rico. Stayed in Ponce and flew in/out of San Juan so did the Hwy 52 in each direction. Def echo the toll pass - my rental car company said the same thing - don't worry about the red toll light. They charged a flat fee ($10 I think) for this convenience and then billed back the actual tolls after the fact. Made the drive much easier and hassle free.
The proximity of Guanica to the wall is what appealed to me. It did not disappoint. I cant speak to restaurants, etc in Guanica, but on the advice of the folks at Island Scuba we went to lunch in Boqueron. There seemed to be a nice assortment of restaurants and things to do, for a small town.