I've been told that these are disconnect diodes. However, I am unsure as to what purpose they serve. They are used on the line that sends a signal to a piezoelectric, the return signal from the piezoelectric returns on the same path but has a RX/TX switch that doesn't allow the initial signal in.
simulate this circuit – Schematic created using CircuitLab
In this application, the forward voltage drop of the diodes means that they have a relatively high impedance for voltages less than about ±0.65V, which effectively disconnects the source impedance of the pulse driver from the transducer when receiving, preventing the driver from "loading down" the high impedance of the sensor and reducing the overall sensitivity.
When transmitting, the drop is negligible.