What Animal Am I?

Clue 1: I am a large land animal and I live in Africa.

Clue 2: I am an animal which likes to swim or lie in lakes or rivers for most of the day. It helps to keep me cool.

Clue 3: At night I leave the water to eat my favourite food which is grass.

Clue 4: I am an animal which looks quite gentle, but I am really fierce and dangerous.

Clue 5: I am an animal which can stay under water for a long time by closing my ears and nostrils.

Clue 6: I am famous for my enormous yawn. When I yawn you can see my huge teeth.