I don't know why everyone is harping on the "choice" of no radio. Without a permanent tether there couldn't be any voice communication. Salt water is a ridiculously poor medium to transmit EM waves, to talk to a relatively shallow military sub you need such long waves that it takes multiple seconds to send a single letter, at 4000m you ain't transmitting shit.
As for navigation, that isn't trivial. No GPS signals getting there, you're reliant on charts, inertial sensors, and some sort of speed measurement system