Sorry it's taken me so long to review this, Bine. It's a lovely poem capturing both the despair of Azkaban and Sirius's relief at being free.
I love the repetition of 'I swim' which really aids both the flow of the poem and also Sirius's escape as Padfoot paddles to the shore.
The only things I'd quibble are
Harry, I shall see you soon.
I'd change the 'I shall' to 'I'll' because I think it scans a little better. The only other thing is the exclamation mark at the end of 'Sea' which I'm not sure is necessary, but that's a personal preference.
I like the way you've crept into the mind not just of Sirius, but Padfoot. There's a distinct shift in mindset which is very clever. Well done.