Why southern hemisphere is colder

Why southern hemisphere is colder than northern hemisphere?

Southern hemisphere is colder than northern hemisphere. There seems to be no clear understanding about this phenomenon, according to a new video. 

I think the reason is quite straightforward. Southern hemisphere has a lot more ocean. Sunlight penetrates into the water. Energy is distributed over quite a depth in the ocean. With less energy at the surface, Southern hemisphere is colder. It is that simple.

Is there any problem with my reasoning?

https://youtu.be/Gn39FX4m7fM