Underwater optics is a subject area with many applications from vision to remote sensing. Most have traditionally suffered from a limited range capability, despite advances in compact laser sources and beam steering technology. One reason for this is a lack of fundamental data concerning the propagation characteristics of high-power cw and pulsed beams through turbid media at the distances required. This project was designed to address that shortcoming by investigating two non-linear effects of high-power lasers underwater: thermal lensing and stimulated Raman scattering (SRS). A novel test facility was designed and attenuation measurements made at pathlengths up to 27m, using an array of mirrors, with argon ion (4.5W, 514.5nm) and Nd:YAG (140mJ, 532nm) lasers.