The main concept artists for the base game were Cory Strader, the art director, and Pat Presley, who is a freelance artist also known for his work on the Star Wars: The Clone Wars series as well as several other outsourced artists.
For Subnautica: Below Zero, the role of art director is once again filled by Cory Strader who has drawn much of the story and landscape art, Pat Presley has also returned as an artist and Alex Ries has been hired for the role of creature design. Several other artists have been outsourced from other companies such as Fox3D, notably Dima Pertsula who has created several highly detailed landscapes.
Concept Art is made to showcase future features of games, although the designs are subject to change during the course of development. Not everything that is shown in Concept Art is guaranteed to make it into the game.
According to developers, the small vehicle seen in "Cave Thing" was meant to be deployed by larger vehicles to support the player while swimming and was likely going to be unmanned. 
The concept art "Interior Sketch" shows the interior of a vehicle that was intended to allow placement of modules at predefined points in a predefined chassis, including "airlocks", "item fabricators", and "power systems", which later became the Cyclops. 
The words "Flayra 2" can be seen on the diver in the "Interior Sketch" concept art. This is a nod to Charlie Cleveland, one of the developers of Subnautica.