what was the cause of the captain's death ?

The cause of Billy's death was a stroke.

Even as he did so, he reeled, put his hand to his throat, stood swaying for a moment, and then, with a peculiar sound, fell from his whole height face foremost to the floor.

I ran to him at once, calling to my mother. But haste was all in vain. The captain had been struck dead by thundering apoplexy.