Agree with the above. What was required in our day was ability to swim 4 strokes: breast, back, free and side. For breast, back, and side, you were measured on how few strokes it took you to cover a fixed distance. Thus, technique was more important than speed. For freestyle, speed determined your grade.
We also had to tread water in clothing for 15-20 mins, jump from the 10M platform, swim underwater for 25-40M (can't recall the distance), do the 40-year swim (swimming constantly for 40 minutes in khakis, no shoes and graded on distance covered), and probably some stuff I no longer recall.
IF you've ever taken basic swim courses at the Y or your local pool, you can probably get Cs or Bs in USNA swimming, unless the standards have changed. If you've ever swum competitively (this does NOT mean you are currently a varsity swimmer), you can get Bs and maybe As. If you've never taken swimming, you will lack the technique and speed to score well -- unless you are lucky enough to be a naturally gifted swimmer.
Thus, if you're not comfortable in the water and/or don't have good technique, a swim class or two is time well spent. It's not necessary, but it will help you over the course of your 4 yrs at USNA.