Knowledge of all factors that must be taken into consideration for in-water acoustics, to include distance, sound speed in the water, sound velocity profile, absorption, spreading loss, reflection, sensitivity, and receive/projection patterns of hydrophones and sources. Ocean engineering of complex in-water arrays, in-water installation of arrays in harsh environments, a thorough understanding of acoustic measurement system calibration (both electronic and in-water), array validation, and maintenance of in-water arrays.