Floorguy had it right. First you have to know if there is sealer on it or not.
If not, then there are few NEW things that you can use.
First, a Consolidator is in order. This hardens the concrete and if it is a salt water pool helps retard the damage from the salt.
Second, a Color Enhancer sealer will work. We use Crystal Magic Consolidator and Crystal Magic Color Enhancer.
Now, the neat stuff is Crystal Magic Acid Proof, available in Polished, Matte or Slip Resistant. Instead of the above. This is a permanent sealer that is completely acid proof, oil proof, water proof, matter of fact the only way to get rid of it is to grind it off. It ALWAYS color enhances, which, depending on how you look at it, is good or bad. It can also be used as a permanent graffiti preventer, as the inks will just run off it! Pricey, but properly applied, the client will love you forever.