Sound waves. Acoustic waves travel much farther and there are transmitting stations underwater. Contrary to the other reply, ELF is not used and is impractica

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 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-antenna-a-key-system-of-modern-submarines/ https://www.navalnews.com/naval-news/2022/04/communication-a...