When I was growing up with Bionicle, I mostly followed through the comics. Yes I watched the films and owned some of the early books, but didn't read the books and I didn't think every aspect of the movies was canon. I also played the console games, but again I knew not every detail lined up.
At the time of releases, I didn't understand the transition from 2006 to 2007 storyline, and I don't recall it being depicted or explained in the comics. So I didn't quite understand how the Mask of Life went from Vezon to the Toa Inika, and then into the ocean.
As a kid, my headcanon explanation was that it was either lost in battle, or more humorously, one of the Toa carrying it off the island tripped and dropped it in the surrounding ocean.
I still don't really know if it was explained, and if someone knows, I'd appreciate it.