In herman melville’s mobydick, what important idea is stressed by ahab’s drowning as a result of becoming entangled in the harpoon line?

The idea that the obsession can destroy you.
One of the themes of Moby D.ick is the dangers of obsession be that obsession out of love or out of hate. Ahab's quest to hunt the whale was a clear case of obsession, and he abandoned everything for it. In the end it cost him his life. There is also a bit of irony in that that it was the weapon which was meant to kill the whale eventually led to Ahab's death.