Answer:
I am the yellow hem of the sea's blue skirt.
Man walks over, and man swims under. In times of war, it can be burned asunder. What is it?
Pirates put their booty in it.
A beggar's brother went out to sea and drowned. But the man who drowned had no brother. Who was the beggar to the man who drowned?
The stack just might be sent all over. Full of what's new, yet it's nearly obsolete.
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.