Is Melville really generalizing in Moby Dick when he says all men desire going to sea at some point in time? Prove him wrong if you'd like!
If we view the sea as a metaphor for the absolute, I would agree with Melville's assertion. Most people, I believe, crave a numinal experience, want to encounter the universal or absolute directly in a sort of soul-to-oversoul tete-a-tete.
An argument can certainly be made that the sea represents, in Moby Dick, more than a body of water. As pointed out above, we can look at the sea as the place for adventure or as a metaphor for the grand stage where life takes place in all its complexity. I'd suggest that we can also look at the sea as a symbol for unencumbered nature; an embodiment of an unmanifested god-head wherein swims the manifested god-spirit, Moby Dick, the arbiter of fate.
For Melville the sea represents life. As critic John McWilliams writes in his essay "The Epic in the Nineteenth Century," published in The Colombia History of American Poetry,
...readers approaching the novel for the first time are advised to consider it both as a work that portrays life on a whaling vessel and as a literary investigation of the conflict of humanity and fate.
Melville himself declared that "a whale ship was my Yale College and my Harvard." So, just as he had escaped a grim life of poverty by taking to the sea as a cabin boy and finding adventure, he learned many of life's lessons. Melville felt that others, too, would desire to understand the metaphysical lessons that he had learned at sea on the whale ship that was his college.
To me, Melville was correct to some extent, but not completely. And I think he is not correct any longer now that ocean travel has lost much of its romance.
I think that he is right in the sense that all people want adventure. The ocean is a place where a person can feel challenged. You feel close to nature and you feel like you really could be in true danger (which is why it is a challenge). All the times that I have been on the ocean it has been on an 18 foot boat and you really get the feeling of how powerful the ocean is. People going to sea in Melville's day would have felt that too in their relatively small ships that depended on the wind for power. Nowadays that feeling is not so present since most people who go to sea do it on huge ships that are powered by motors.
Anyway, going to sea represented adventure and a challenge of a man's masculinity, especially back then. I think that this is what Melville is talking about.