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