This paper investigates the eavesdropping attacks in underwater acoustic networks (UANets). In particular, we propose an analytical framework to model the eavesdropping attacks in UANets in terms of the eavesdropping probability. Results of extensive simulations match the analytical results, indicating the effectiveness and accuracy of our model. Besides, we also find that the eavesdropping probability in UANets heavily depends on the underwater acoustic channel characterizing by the acoustic signal frequency, the spreading factor and the distance between the transmitter and the eavesdropper. This study provides useful insights that will help us to design anti-eavesdropping schemes with a better protection on the confidential communications.