The crust below the oceans is

made of sand and salt sediment

mostly granite

made of metamorphic rock and granite

basalt and a layer of salt sediment

The crust below the oceans is thinner than the crust below that continents.

This crust is called oceanic crust. It is predominantly made up of mafic magma which produces basalt and other mafic rocks when they cool and solidify.

The oceanic crust is generally thinner than the continental crust below the continent.

The oceanic crust is about 5-10km thick.

Continental crust is about 30km thick.

In terms of density, the oceanic crust is denser than the continental crust.