Why is the density of oil a key factor with oceanic spills and fire?

I can't find the answer in my book

I'm no expert and I'm just gussing right now, but I think its because its how oil floats. When you pour oil in a glass of water, it floats. Same with the oil in the tankers, and because they float on top of the water, it is hard to keep the boat in the water still.

I think I'm wrong though.