Its not a documentary, its a propaganda piece.
Victory at Sea presents little to no new information or insight into the War. While it does contain a large amount of footage, the footage is badly edited, without label, and without source.
If you want to watch many hours of things exploding, set to music and (somewhat pompous narration), this is a very very good series. If you are looking for information or insight into WWII naval battles, there are a number of more in depth documentaries, or semi- documentaries like 'Tora! Tora! Tora!' or 'Midway.'
Also, I have heard comments from several people on-line that the audio is poorly mixed in some versions, and that the Music drowns out the (already minimalist) narration. I didn't have a problem with this, but beware.