水泳 and 泳ぐ: a learned activity and human or fish movement
Compare swimming basics and weekly competition practice with a summer habit and fish swimming at a past moment.
水泳: learned basics and weekly practice
水泳 (すいえい) means swimming. 「夏休みに水泳の基礎を習いました。」 means “I learned the basics of swimming during summer vacation.” The learned content is swimming basics during summer vacation, without a mastery or prize result.
「弟は水泳大会に向けて毎週練習しています。」 means “My younger brother practices every week for a swimming competition.” The younger brother practices weekly for a competition; its result is not supplied.
泳ぐ: a summer habit and fish in motion
泳ぐ (およぐ) means to swim. 「夏になると近くのプールで泳ぐ。」 means “I swim at a nearby pool in summer.” The sentence describes a summer swimming habit at a nearby pool.
「魚の群れが透き通った水を泳いでいた。」 means “A school of fish was swimming through the clear water.” 泳いでいた describes a school of fish swimming at that past moment.
Keep learning, practice, and movement distinct
水泳 names the activity learned or practiced; 泳ぐ expresses the subject's movement.
The fish's movement cannot become completed human training. The younger brother's ongoing practice is a separate report.
Does the fish example with 泳いでいた say a person has finished swimming training?
Show the answer and why
No. It describes fish swimming at a past moment. Its subject and time scope differ from learning basics or practicing for a competition.