This is what's left of a seven-arm octopus (Haliphron atlanticus). We came across a couple of these while in Ogasawara.
Seven-arm octopuses are the largest known octopus species, allegedly growing up to something like four metres in length and weighing 75kg! The ones we saw were from significantly smaller animals, as you can tell from this photo of my friend Tomoko-san photographing the floating blob:
Despite the common name, this octopus has eight arms, just like all other octopuses. The "seven-arm" name derives from the fact that the mating arm is coiled away and obscured from view in males...often giving it the appearance of having one less arm than it should.
In case you're wondering, the octopus was slimy, and left behind a sticky film/ residue if you touched it (Of course...I had to touch it!). Otherwise, it was gelatinous and slippery, similar to what the bell of a large jellyfish feels like (Yes, I've touched those too).
I'm not sure if anyone knows for certain, but I suspect the bits we found floating at the surface were leftovers from sperm whale meals, or maybe from other deep-diving whales. Perhaps these octopuses don't taste very good, and the whales spit them out? (No, I didn't taste one).
Note: Incidentally, the plural of octopus is octopuses, not octopi. The word derives from Greek (okto+pous), not Latin, so the Latin plural form (ending in "i") isn't correct. Octopodes is ok too, but no one uses this word.