Back in September 2013 there was a massive spill of molasses from a pipeline owned by the shipping company Matson. The pipeline was used to load cargo ships bound for California. The pipe leaded as much as 233,000 gallons of molasses into Honolulu Harbor, killing pretty much everything in it's path. Molasses is more dense than water so it immediately sank to the sea floor, suffocating everything in its path. Keith Korsmeyer, a marine biologist at Hawaii Pacific University in Honolulu, has also suggested that the dissolved sugar in the molasses creates what's called an osmotic effect, whereby the water inside living cells diffuses outward to equalize the concentration of dissolved substances. He suspects this phenomenon caused the initial massive fish die-offs by dehydrating the fish.
Molasses is both better and worse than an oil spill. Because molasses sinks, it causes immediate mortality, unlike oil which floats and tends to cause deaths more slows. Because molasses is basically sugar, it provides a ready food source for bacteria, which multiply rapidly, depleting the water of oxygen. This creates a localized dead zone, but also breaks the spill down. On the other hand, there are no toxic components to molasses and there should be no long term impacts like the mutations and lowered birth rates seen after oil spills.
On the downside - last month there were reports that the molasses spill has killed thousands of colonies of coral, potentially resulting in the destruction of entire ecosystems.