Unknown Worlds describes Subnautica as a blend of role-playing and exploration. A reductive but maybe more effective way to describe it is an underwater Minecraft. It doesn't look like it will allow for Minecraft's level of customization, but it focuses on non-violent exploration of an underwater world, allowing you to design, build, and crew a submarine. Unlike Minecraft and its simple presentation, part of what makes Subnautica interesting is how it will render colorful coral reefs, strange sea creatures, and other sights that make for a great nature documentary. Judging by the first batch of screenshots, it looks like Unknown world is off to a good start.
Unknown Worlds stresses the point that these are taken from a pre-alpha build, and hence don't represent the final product at all, but the game looks pretty good already. These two are taken from the Koosh Bush Zone, named after those large purple plants, and the Grassy Plateau area:
And this one is of the Sea Glide, a personal water vehicle, and Cyclops, the first submarine to make it into Subnautica.
If you're interested in Subnautica, Cory, Evan, and Tyler talk about what they saw of it at PAX East in this video .