How do I get to the island point of interest in Kessex Hills?
That particular Point of Interest gave me some trouble as well. Luckily, one of my guild mates helped me out.
There is a cave on the adjacent island that you must swim into to reach your point of interest. It should be in the general vicinity of the heart on that same island, as illustrated by this freehand circle.
Additionally, this YouTube video, courtesy of sjohnston illustrates it very well: