What is the density of heavy fuel oil bunker?

In a technical sense, any fuel used by ships is bunker fuel. As regards heavy fuel oil, it's either No. 5 or No. 6 fuel oil, and is most commonly No. 6 or "bunker fuel" or "navy special fuel oil" or something similar.

No. 6 heavy fuel oil has a specific gravity of about 0.94 to 0.95 at room temperature (68 degrees F), and that translates to a weight density of about 59.3 to 59.9 pounds per cubic foot. Water weighs 62.44 pounds per cubic foot, by comparison at 68 degrees F.