Sea longing: It has been suggested that it represents a longing for the past - when all Elves dwelt in Valinor (because Ulmo's voice is heard as the music of water); also that it may be a metaphor for death-longing, when the burden of life becomes to heavy to bear - or, the opposite, a longing for immortality.
Here is a thread where you can find these ideas, and more:
sea longing