Though it is very difficult to clean oil spill but lower density of oil than water is bit advantage. Oil has less density than water thus it floats on the top of water makes it easy to clean. Lets suppose oil has higher density than water then it will be set down underwater and it will be very tough to clean. Oil is non polar and water is polar molecule so they are immiscible and since oil has less density than water it floats on the water which makes it easy to suck the oil in cleaning the oil spill.
For example thermocol pieces on water is easy to seperate since thermocol has less density than water and it floats over water so it become easy to seperate.