Why do sea mammals die on land, and drown on sea?
Mammals use lungs as their respiratory system. These animals in the process of evolution left land and started to live in the water, they developed organs that are used to swim. The shape of body also changed to reduce the resistance of water. In spite of all this they didn't developed gills. 

Lungs can help in respiration only in air. So the mammals have to come above water in order to take air. You correctly have noticed that these animals die if they can't reach air, i.e. they die of drowning. 

But one thing I want to mention is that they don't need to reach land in order to take breath. They just need to swim up to the surface of water for this purpose. 

Anyway, I think you got the point. Feel free to ask if you have further confusion on this topic. I shall explain more if needed.