In Underwater Acoustic Sensor Networks (UASN) designing Medium Access Control (MAC) protocol is a challenging issue. This is due to low bandwidth and high propagation delay in UASN. This paper proposes a MAC protocol for UASN called Aloha with Virtual Back off Algorithm (Aloha-V). The VBA approach is the collision avoidance mechanism for wireless networks. In this proposed work we use VBA with non-counter sharing. The Aloha-V MAC protocol reduces collision probability thereby improves the system throughput by restricting the number of transmissions carried out by the node. The Aloha-V protocol is compared with the existing underwater Aloha protocol. The simulation results show that the proposed method reduces the number of collisions and significantly increases the throughput compared to classical underwater Aloha protocol. © Research India Publications.