The mask is a non aquatic version which is the only available Arthron in studio. I gave him an underwater surfboard to honor Lhikan and Tahu, his two greatest heroes. You can take apart the board to give him two swords as well.
Nice one. We have Toa with lava boards and sky boards but no actual normal one. From where are those Inka torso armor and Cordak blaster?
A custom stud.io parts pack called Biopack. It should be on the bionicle subreddit.