Here's why:

* Density is a measure of how much mass is contained in a given volume.

* Gasoline has a density of roughly 0.75 grams per cubic centimeter (g/cm³).

* Seawater has a density of roughly 1.03 g/cm³.

Since seawater has a higher density, it means that a given volume of seawater will weigh more than the same volume of gasoline. This is why gasoline floats on water.