Umm, there were actually four of those levels in World 8...
The first level had tanks...
The second level had ships (boats)...
The last level of the second area had airships...
And the penultimate level was similar to the first, but also had ships at the top of the screen.
But...yeah, I could definately see one of those being a level. Except for the fact that you can get caught in between the larger tanks at high percentage without dying, and (for the first and fourth example) bottom-screen KO's are nonexistent. Those aren't really big problems though, if you think about it.
Oh, and...it's called Super Mario Bros. 3. Super Mario World is a different game. I swear, the number of people who can't tell two Mario games apart...disappointing.