Personaly, cant imagine swimming with no music, it feels like something is missing. Heres my favs:
"Underwater Love" by Smoke City.
"Oceans" by Hillsong UNITED.
"Swimming Pools (Drank)" by Kendrick Lamar.
"September" by Earth, Wind & Fire.
"Budapest" by George Ezra.
"The Ocean" by Mike Perry ft. Shy Martin.
"I'm on a Boat" by The Lonely Island ft. T-Pain.
These are just a few suggestions, but ultimately the best music to listen to while swimming in the pool is whatever makes you feel relaxed, energized or happy.