To me, this creature is an amphibian whose skin isn't permeable enough to absorb dissolved oxygen from the water. Despite that, it spends a great deal of time in water. I don't know whether it is swimming for the fun of it, or if it is searching for anything. When not underwater, I imagine him often sitting atop a device that resembles a floating aquatic plant. I also imagine said device having a camera on its underside. His eyewear gives him the option to see what the camera sees (translated to his eye position, of course.)

Being like most known amphibians, he cannot safely exist in water with high salt content (unless he covers his body completely.) Since his skin isn't as permeable as that of other amphibians, being exposed in salt water is SLIGHTLY less dangerous. The highest speed he can hit underwater is about 24 mph; not particularly fast or slow. He can maintain this speed for nearly a minute. At max, he can remain submerged for ~10 minutes before resurfacing.

The plant directly beside the creature is an aquatic plant I made up. It floats freely until it is ready to release its flowers, after which it sinks and anchors itself to the lakebed. The other two plants are both real ones. The one at the upper right is a water milfoil. Have a blessed day, all of you!