There's a couple of different "wake up" signals that can reach deeper into water. Their biggest limitation is very low bandwidth, so an attack sub will emerge (/send up a buoy on a tether) to get an updated tactical map.
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.