Also highlighting the E-6B Mercury (and the upcoming EC-130J), which among other communication options has a 5-mile (!) VLF antenna it deploys vertically in midair (!!) to establish limited-bandwidth communications with submarines.
https://m.youtube.com/watch?v=en-qekZX4ws
But I believe that's only good down to ~60 ft. Anything deeper requires the really long land-based ELF arrays.