Take swimming lessons. Some people take to the water naturally, but most do not. You may well do under the water but sooner or later you are going to be on top of the water and potentially with a buddy needing your help. N
To that point... here's some data from a study I did recently on scuba training with ~700 divers. One of the things that was most highly correlated with student-reported "Preparedness for OW" and "Satisfaction with Pool Session" was the student's self-reported swimming ability coming into the pool sessions:
Getting New Divers Off On The Right Foot
By comparison, here are the "Preparedness" and "Satisfaction" scores for people with self-reported swim ability of "Better Than Average"
80% of those students felt "Mostly" or "Extremely" prepared:
And 76% of them were "Mostly" or "Extremely" Satisfied with their pool sessions...