As mentioned in the comments, there are two sources 1 and 2.
The refrain in the first version is
Flagge am Mast,
Die der Führer uns gab,
Flagge, wir machen dir Ehre,
Engelands Macht
Uns're Stunde die Nacht
Unser die Freiheit der Meere.
Unser, unser,
Unser die Freiheit der Meere.
It has eight verse lines and two rhymes (lines 3, 6, 8 and lines 4, 5).
The refrain in the second version is
Flagge am Mast, die der Führer uns gab,
Flagge, wir machen dir Ehre!
Engellands Macht muß zu Grunde hinab,
unser die Freiheit der Meere!
Unser, unser, unser die Freiheit der Meere!
It has five verse lines and two rhymes (lines 1, 3, and lines 2, 4, 5) where the fifth line appears to be separated from the first four lines. This is more plausible than the first version - especially because all other parts of the text are quatrains with rhyme scheme 1- 3 / 2 - 4.
To clarify which is the correct version listen to this old record. The text agrees with the second version. Therefore the question is based on a false assumption.
However, I observed a weird phenomenon: When I listened to the song, I understood "Engellands Macht muß [...] die Nacht", where I couldn't identify the part [...]. Only when I followed the text while listening, it became quite clear that is "Engellands Macht muß zu Grunde hinab".
And I hope that nobody listens to this song except for historical research interest.