In The Lord of the Rings, Legolas hears a seagull’s call and right away starts wanting to go to the sea. Then there all of these other Elves that are leaving Middle-earth in droves, going to the sea. Many fans have wondered about that and started some fanon based on what they concluded. To them, Sea-Longing is like an unexpected mania that erupts when an Elf has some contact with the sea. In truth, it is a combination of several things, and it is different for the two Elven races in Middle-earth.
The Telerin Sea-Longing
As you should know, there are three races of Elves, the Vanyar, the Noldor, and the Teleri. The Teleri are drawn to water in general (S 52-3). Legolas is a Telerin elf, so when he got the thought of a huge ocean of water in his head, he got very excited. His father lives over a river as well.
The second part of their sea-longing isn’t really sea-longing at all. When the Elves were first awakened, the Valar summoned them to live in a protected isle, away from the jealousy of Melkor. Some elves stayed where they were awakened, their fates, we do not know (S 52). Some Teleri stopped in Middle-earth, where they found nice bodies of water to live by. The rest crossed over
the sea to Valinor (S 53). The Valar wanted all of them to live in Valinor. The call that the Elves in Middle-earth feel is not to the sea but to what lies beyond it. So, Legolas had two things going against him when he heard the cry of a seagull. A Telerin love of water and the original summons of the Valar.
The Noldorin Sea-Longing
The Noldor were banished from Valinor for the Kinslayings of the Teleri of Alqualondë (S 87-8). Their ‘call to the sea’ is actually a wish to go back to their homes in Valinor. They were only waiting out their banishment in Middle-earth; they knew that they could return after an Age or two. Toss out that idea of ‘Oh it so twagic that the Elves are leaving Middle-earth,’ and think ‘Oh it is so wonderful that the Noldor are returning to paradise!’ For them, Sea-Longing is Home-Longing.
The Effects of Sea-Longing
This is where the greatest misconceptions about Sea-Longing are. It isn’t a mania. It is the knowledge that someday one has to go to Valinor. It doesn’t reach up and grab some poor Elf and make him or her forget all obligations with a mad desire to build a ship and cross the ocean. Every once and a while, an Elf will be reminded that there is a journey that he or she must make, being obedient to the Valar. For example, Legolas stayed in Middle-earth long after he was reminded of the need to go. He explored the Dwarven side of Middle-earth with Gimli, helped rebuild Ithilien, and served Aragorn till his death. Legolas stayed in Middle-earth for 121 years after he was reminded (LOTR 1072). No insane gravitation to the sea.
There you have it. Sea-Longing is a generalization for a variety of things with the same result: life in Valinor. It is not a blind obsession or mental illness. It is knowledge. I hope this has cleared up the confusion about it.