The blue–green spectrum is the optical transmission window of seawater, which can be used for underwater laser communication, laser bathymetry, and ocean LIDAR. The attenuation coefficient varies from different types of seawater, and the optimal wavelengths for laser detection in deep seawater are between 420 nm and 510 nm, while the optimal wavelengths for coastal seawater are between 520 nm and 580 nm[1–5]. As is known, an optical parametric oscillator (OPO) pumped by a solid-state laser is regarded as a promising tunable laser source. In particular, a (BBO) OPO pumped by an ultraviolet laser is able to generate a wide range of wavelength emission from blue to near infrared[7–9]. In 1988, Cheng et al. firstly, to the best of our knowledge, reported the BBO-OPO pumped by 355 nm with the signal output tunable from 0.48 to 0.63 µm, where its total maximum energy conversion efficiency was 9.4% when the pump pulse energy was . Later, Fan et al. reported a tunable BBO-OPO design, at 30 Hz repetition rate, using three sets of mirrors so that wavelength tuning range covered from 412 nm to 2.55 µm.