Underwater acoustic sensor networks are quite different from terrestrial wireless sensor networks. Localization for underwater applications is different due to the bandwidth limited acoustic communication, sparsely distributed network deployment, and more expensive and powerful sensor nodes. In this paper, we propose a new scheme to achieve better localization accuracy for underwater acoustic sensor networks. Instead of using the commonly adopted circle-based event detection and least squares algorithm based location estimation, the proposed scheme utilizes the hyperbola-based approach for event localization and a normal distribution for estimation error modeling and calibration. Our analysis and simulation results indicate that the performance of the proposed scheme is clearly better than those from the least squares location estimation based localization schemes.