To me it seems more like the song is talking about some type of disease. The fire of unknown origin referring to both the disease, as well as the fever caused by it. "Like the ocean in a fire, so thick and gray." Often times the skin of a person near death can take on a grey pallor. The song depicts the events as being something that seems to come out of nowhere at all, and afterward leaves the survivor of the deceased thinking there was something that could have been done to prevent it. "I should have seen it coming! I should have made her go to the hospital sooner! There must have been something I could have done..." is a very common line of thought in a situation like this.
To me it seems more like the song is talking about some type of disease. The fire of unknown origin referring to both the disease, as well as the fever caused by it. "Like the ocean in a fire, so thick and gray." Often times the skin of a person near death can take on a grey pallor. The song depicts the events as being something that seems to come out of nowhere at all, and afterward leaves the survivor of the deceased thinking there was something that could have been done to prevent it. "I should have seen it coming! I should have made her go to the hospital sooner! There must have been something I could have done..." is a very common line of thought in a situation like this.