Mermaid’s Lair  is a horizontal cave that houses a fresh water lens.  The opening to this cave is in a small swampy setting within mangrove roots.  Following heavy rains, the water within the swamp is tinted orange with tannin, a natural astringent that inhibits microbial activity, thus resulting in the natural durability of wood.  In plants, tannins are defensive compounds that counteract bacterial and fungi by interfering with their surface proteins and more.  
    The brown stuff that you see in the film stuck to the walls is biofilm or what is known as bacterial fouling.  Amino acid analysis done by Dr. Christian Schwabe identifies this material as a protein, all generated by bacteria.  These systems are formed by bacterial acids.
Diving Mermaid’s Lair on Grand Bahama Island, The Bahamas