Sound waves. Acoustic waves travel much farther and there are transmitting stations underwater.

Contrary to the other reply, ELF is not used and is impractical: https://en.wikipedia.org/wiki/Project_Sanguine

For VLF you can drag a very long antenna behind you, but most submarines just launch a buoy with a normal antenna to the surface and a fiber connection to allow them to stay below the surface.

https://www.navalnews.com/naval-news/2022/04/communication-a...