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.