The difference between the two boys was not overtly obvious.
They had the same dark, wild hair. The signature Black veneer. The same unsettling eyes.

But inside, they were like night and day. Which one was which - nobody could ever say for certain. What could be said was only that Sirius Black, and his brother, Regulus Black could not be any more different.

Years later, if asked about Regulus, if asked about his death, Sirius Black would just stare off at something very vague and distant. Sirius would run and lonely hand through his hair, and shake his head slowly. His unsettling eyes would flash angrily for a second, but then soften with a longing and a surprising remorse. Surprising, of course, because Sirius would sometimes say bitterly that "Regulus got exactly what he bargained for..." ...

But once in a while, if the timing is just right, and the subject approached in just the right way... Sirius Black will shift his feet restlessly, and for a moment look like a teenager again. His shoulders slump down a bit, and he looks utterly and completely crestfallen. And then he'll softly say, "It was like the ocean, you see."

And what he means is this:
Once, for the summer hols, the whole Black family got together with the Malfoy family, and they all made a go of it at a summer resort on the coast. Sirius was just old enough to be coming into his own... but not old enough that it meant anything yet. Regulus, just a couple years behind, was at his side the whole way. Or most of the way, that is.

Because while Sirius was arrogant, stuborn, and fearless... Regulus was utterly petrified of the ocean. The huge expanse of unending water. The limitless possibilities. Neither boy would understand it then, but what Regulus saw in the ocean that summer was the same thing that would haunt him until his death: indecision. Alone and without someone to follow, Regulus instinctively knew that he would flounder. And drown.

And when Sirius tells you that Regulus died... like the ocean... what he means is that Regulus only swam out that night because he thought that Sirius would eventually lead him back to shore. Regulus put his trust in his older brother completely; followed his voice and his goading right down the inky coldness of his overwhelming fear, and all the way out, past the breakers.

Sirius, unafraid of anything, single-minded and sometimes unthinking, didn't pay attention to his brother as he forged his own way. Swam his own direction, and left the younger boy behind. Sirius returned to shore. Regulus, lost in the night, did not.

And for many long anxious moments, Sirius felt the deep, swirling panic that only a brother can know when he looks out into the dark of night, and sees nothing. And strains his ears hard against the emptiness, and hears only the pounding sound of surf.
But when Sirius finally, desperately, saw the pale speck of what was left of his brother - Sirius dove back into the ocean without hesitation.

Regulus nearly drowned that night. And Sirius always, secretly, blamed himself for what could have happened.
And, in the end, it was like the ocean.