Yeah, my basic understanding of submarine communications is that lower frequencies penetrate the water better. Lower wavelength needs a longer antenna. The system US subs use is a very low frequency from what I understand.
Regarding "longer antenna" for submarines... -- I recently learned about this signal from https://www.sigidwiki.com/ -- which has been helpful to ID all the fun stuff you can see with RTLSDR
Corona, stuff like this, the sheer gall, it's impressive.