Underwater Wireless Sensor Networks (UWSNs) provide a solution for monitoring those unstable environments which is considered frightening for human presence. These networks are mostly formed by acoustic sensor nodes and surface sinks called buoys that are connected to any onshore control centre. The reliable delivery of sensed data to the surface sink is a challenging task as compared to forwarding the collected data to the control centre. Acoustic channel characteristics create many problems like, low available bandwidth; large propagation delays and high error probability that restrict the efficiency of UWSNs. Beside these, availability of limited resources and continuous node movements are major threats for the reliable data deliveries. With these constraints, it is a difficult task to design a protocol which has the ability to maximize the reliability of these networks. In this paper we provide a reliability model in order to insure the reliable data deliveries to the surface sinks. For this, we proposed two hop acknowledgment (2H-ACK) reliability model, where two copies of the same data packet are maintained in the network without extra burden on the available resources. Simulation results show that 2H-ACK can achieve better delivery ratios as compare to traditional hop-by-hop acknowledgment reliability without additional resource consumption. © 2010 IEEE.