Muir can, at will, transform his body into an equal amount of salt water (roughly 200L). While in this state he is still capable of moving, either over a surface or through another liquid. He can also manipulate small amounts of salt water seperate from his body, but not while he himself is in water form.
The barnacles/sea-life pictured on Muir's body are an extension of his magic, and will quickly die and turn to salt water if removed.