As an ancient mariner, I spent some brief time standing sonar watches in a destroyer. It was important to know the sea's thermal profile vs. depth, which affects the transmission and refraction of sound through the sea. For that, a bathythermograph device was lowered into the sea. Technology has undoubtedly changed, but I wonder what devices can measure the temperature profile down to, say, 4,000m? Or maybe, past a thousand feet, or so, it doesn't matter that much?