Thirty, English Beat Poetry

Walking down the riverbank
Counting clustered swans
It's dark and wet
I'm cold and weary
My score reaches twenty nine
But I'm not sure of thirty
Hey, thirty, my proud bird
Are you there?
Why are you submerged?
It enters my head
That thirty's dead
And that's why, then
The twenty nine living swans
All of them
Have their heads underwater
Sunken for thirty
Serene swan grandmother
Thirty, who is no more
Thirty's gone to swim
In a deeper river.