The base color you use is largely dependent on what kind of lighting conditions you wish to portray. The cobalt blue might work well for a sunlit mid-day in the Pacific, less well for an overcast or stormy day in the North Atlantic. (The surface of water is basically a big reflector of sorts, simply put.)
That said, your base color in and of itself is less important than the variations of color seen in nature. Google "ocean water" images and you'll see what I mean. It's all in the shadows and highlights, wave patterns, even spray (if present). It'll take a bit of practice, but you'll get there.
Good luck, and have fun!