In this paper, we present a novel technique for localizing an event of interest in an underwater environment monitored by an underwater sensor network. The network consists of randomly deployed identical sensor nodes. Instead of proactively localizing every single node in the network as all proposed techniques set out to do, we approach localization from a reactive angle. We reduce the localization problem to the problem of finding 4-Node Coverage, in which we form a subset of nodes such that every node in the original set is covered by four nodes belonging to this special subset - which we call the anchor nodes for simplicity. This subset of anchor nodes behaves like a backbone to the localization process. The anchor nodes are localized based on an "underwater GPS" preexisting system. Whenever a node detects an event, it is reactively localized using the anchor nodes, and the sink is supplied with the necessary information. By limiting the sensing range of the sensor nodes, once we have obtained the location of the node that has detected the event, we have a rough estimation of the location of the event. We show that in terms of energy consumption, this localization technique far surpasses others in terms of energy efficiency.