Okay, the salps were kind of creepy. The colony was like some kind of giant snake with thousands of tiny white mouths opening and closing…
For the original question- not a vertebrate, but a species of ribbon worm, the bootlace worm, has been recorded as being longer than a blue whale.
The longest non-whale living verterbrate is probably either the whale shark, or the oarfish species, the fabulously named King of Herrings which have been apparently recorded at sizes longer than any whale shark (though they are thinner, so not so heavy), but very little is known about them. There could be anything in the depths still really…
The Portuguese Man o’ War can be longer. But while it [del]walks[/del] swims like a jellyfish, and [del]talks[/del] floats like a jellyfish, it is actually not one animal but a colony of organisms, so is cheating more.
Nice username / topic combination. 
Isn’t each individual strand of a Man-o’-War a single organism, though? So you could still say that one of the strand-organisms is the longest.
Those poncy bastards float on the surface all purple. Sure there is a bunch hiding, but it’s like they’re advertising.
They’re four types of polyp. You might be right, the killing strands are dactylzooids and are most of the length, not including the sail polyp (the other two are feeding and reproduction, and I guess intermingled in the tentacles).
… “thousands of tiny white mouths opening and closing” …
Oh, now that is getting a bit Lovecraftian, indeed! Interesting!