My snorkeling buddy spotted this rare nudibranch in about 1 metre or 3 ft of water along a rocky, algae and sand covered reef in West Maui.  Needless to say, we were pleasantly surprised to see such a large (3 inch) colourful nudibranch cruising about in the brilliant morning sun.  The only other red nudibranch, with yellow margin we had come across was the Spanish Dancer, but this  individual lacked the noticeably white lacey bits But apparently they are diurnal, though rarely sighted Red Dendrodoris or Dendrodoris rubra  http://seaslugsofhawaii.com/species/Dendrodoris-sp1-a.html