Black-throated divers are large birds, almost as big as geese. They have small, narrow wings, and thus take off can be difficult - especially if it's your first time.
A long run up is necessary, but it doesn't always work. It will take a lot of practice and the first few flights of any bird tend to look ungainly. The first landing is always problematic, and this particular diver manages a spectacularly clumsy splashdown.