The various shades of blue indicate water depth. The lightest blue is waters up to 50' deep. The next level is up to 100' deep. The last two colors are waters up to 250' and 500' deep. Reefs are quite common in the Pearl Islands.
Letter A: Location of the Duck Blind and the Lost Nithian City of Menkara.
Letter B: Location of Deedra's Merperson home.
Letter C: Location of the 4 Thyatian Supply deposits in the Pearl Islands. The one furthest East has been captured by Jylseara's forces.