Abstract: Sensor and communications systems for communicating measurements from subsea equipment, such as at an offshore well, to the surface. A sensor for a physical parameter, such as pressure or temperature at a blowout preventer, capping stack, or conduit in communication with the same, is electrically connected to a subsea acoustic transponder. An acoustic monitoring transponder deployed near the well periodically interrogates the acoustic transponder with an acoustic signal, in response to which the acoustic transponder transmits an acoustic signal encoded with the measurement. The measurement data are stored at the acoustic monitoring transponder. An acoustic communications device later interrogates the acoustic monitoring transponder to receive the stored measurement data for communication to a redundant network at the surface.