What Am I?

I am a dolphin, 
Gliding through the waves,
I swim up,
I swim down, then I swim all around
I am some driftwood,
Detached from my tree,
I fell into the river,
And floated out to sea,
I am the moon,
Casting light across the vast sea
As I watch the sun rise,
I realise,
My time to watch the earth is over,
So, I sigh, turn, and leave, 
only to come back soon.