Our three German Shepherds all swim in our vinyl lined pool. So do all visiting dogs. Some of the situations you'll be dealing with:
1. Entry and exit
Dogs normally exit a body of water at the same point they got in. Lead your dog to the steps and teach him that's the way in, then guide him back to the steps so he can get out. GSDs are smart. He'll figure this out the first day. Then, even when you toss a toy and he dives in from the side, he'll swim to the steps for another go.
2. Dog hair
We don't call them 'German Shedders' for nothing. No matter how well groomed your GSD may be you will have a lot of hair floating on the surface after a dog swim session. Run your pump during a session and for a couple hours after, put a skimmer sock in the basket, and check it after 30 minutes. I found the PoolSkim to be much more effective for catching and trapping dog hair than the skimmer is. What doesn't get caught will be bunched up on the bottom the next morning. It's easy to scoop out with a net.
3. Chlorine and wet dog
Hose him off with fresh water when he's done swimming for the day, then dry him with a dog dryer. Because GSDs have an undercoat their fur will never really dry, especially if you live in a humid climate. If you don't dry him he will eventually get moldy and you'll know it by the smell. Fungal skin infections are a Bad Thing so don't let them happen. A regular hair dryer is not powerful enough, you'll need to get a dog dryer. Blow drying a grown GSD takes about 25 minutes.
4. Avoid bloat
Bloat and torsion is a life threatening condition and while nobody yet really knows its cause it is thought that excessive ingestion of air plays a role. Don't allow him to bark while swimming. The combination of swimming, barking, and retrieving toys causes them to swallow air and water which may contribute to bloat. Don't feed him 1 hour before or after swimming (or any other heavy exercise).
5. Control access
Once your dog has been in the pool he will always want to be in the pool. Make sure you have a dog proof fence and a locking gate with a latch that he can't figure out. Really, don't underestimate the reasoning ability of your GSD. Unless a latch requires opposing thumbs to operate, he'll get it open. Drownings happen. Don't let it happen to your dog.
Contrary to human swimmers, dogs don't pee in a pool. Their bodies don't work that way, so sanitation is not an issue. And, if friends ask to let their dogs in, you may want to require that the dogs are well groomed (brushed out first) and that the friend goes in the water with the dog. You'll be surprised how many people won't want to do that.
Have fun. My dogs are my favourite pool toys!
AnnaK