The spa pool (thalassotherapy pool) is not for swimming and once you are in it, you won't want to. The water is full of strong jets, pushing your body. There are wwaterfalls falling on your shoulders. There is a contoured metsl (bed), made of pipes set close together. There are thousands of tiny little holes in the pipes and air is forced through them. When you lie on this, the air floats your body up. It is sheer heaven. Infinity will have a second pool with a retractable dome over it, however and although Grand Princess has the only onboard swimming pool I've ever been in that was really designed for swimming, you can do a few short strokes in the others.
It's actually called a "Thallosotherapy Pool" and it's just wonderful. It's very large, but not intended for swimming. You move from one area of the pool to another as strong jets of water massage various parts of the body. Like getting a free massage. No charge for this.