Underwater acoustic sensor networks (UASNs) have recently attracted considerable attention because of the numerous underwater applications. The propagation delay, however, is the most challenging factor for successful communication in UASNs. This article presents a cooperative communication scheme for reliable transmission in UASNs. To cope with packet collision caused due to propagation delay, we propose an adaptive control packet collision avoidance (ACP-CA) scheme to avoid collision of control packets during the communication channel reservation phase (CCRP). Moreover, to reduce a collision between data packets, we propose a relay selection scheme, called the shortest propagation delay-based relay selection (SPD-RS), to select the best relay node (R) for re-transmission in the data transmission phase (DTP). We evaluate the proposed scheme performance in terms of network throughput, average packet success rate, average packet drop rate, and latency. Compared to opportunistic cooperative diversity (OCD) and conventional medium access protocol (CMAP), the proposed scheme performs remarkably well by improving the network throughput and packet success rate, reducing the packet drop rate, and minimizing the latency.