The crust of continents and ocean are made of different kinds of rock. 

They vary in thickness and conduct heat flow differently.

 

Oceans are basaltic. Ocean crust is only 0-10 km thick. This is much thinner than continental crust. The thinner crust, along with relatively rapid hydrothermal circulation, allows more of the Earth's interior heat to conduct through.

Continents are granitic. Continental crust is 35-70 km thick. This is much thicker than oceanic crust. The thick crust acts like an insulating blanket, reducing heat flow from the interior to the surface.