ながきよの とうのねむりの みなめざめ なみのりふねの おとの よきかな。
Nagakiyono tōnonemurino minamezame naminorifuneno
otono yokikana.
To solve this, I copied and pasted na, naga, nagaki, gaki, and so on into Breen’s dictionary, seeing what that produced. Then I selected the most likely answers. Give it a try, if you like. Or look at the next link for my findings.