While it's true that cephalopods use their ink to escape potential predators, so too do humans use their voices to elude danger when they scream; such behavior does not preclude the fluid's use for more complex communication. In his treatise on the vampire squid, philosopher Vilém Flusser discusses this phenomenon while examining the cultural realities of cephalopods as a whole:
“According to popular opinion, octopuses deploy this floating cloud of ink, which they shape into their own image, simply to mislead their enemies, but there is more to the story. Closer observation… has revealed that the act of sculpting the sepia cloud has nothing to do with the enemies, and that, beyond self-portraits, they fabricate countless other forms that are indecipherable to us.”
In other words, there is evidence for a more sophisticated form of octopus language, despite scientific fixation on their liquid howls.
Dr. Scipio Roberts at the University Beneath Chicago has been attempting to decode this hypothetical octopus grammar for many years, and has discussed his work in an interview with the Chicago Entropy Journal (a scandalous matter which resulted in his banishment from ever being published in Nature again):
"Imagine having the intellect to solve an entire crossword puzzle in only a handful of seconds. You couldn’t accomplish this with a pen; you would need some form of spray nozzle, with sophisticated tools to quickly guide it into well over one hundred separate compartments. Now, imagine that crossword puzzle is not merely, two-dimensional, but three-dimensional, and every combination of letters traced from left-to-right, backward-to-forward, and top-to-bottom spells out a grammatically and lexicographically correct word or phrase. Now, imagine seeing this crossword puzzle solved before your eyes, then memorizing that solution in only a handful of seconds, as it will immediately dissolve after being produced.
This is the nature of octopus language: sudden, profound, extraordinary bursts of interrelated information. Hundreds, if not thousands of words are written in mere seconds as a solitary black firework, read in an equally short amount of time, then lost to the currents of the sea. Every member of their species has their own sophisticated personality and cosmology, expressed in a handful of sudden and beautiful eruptions, all of which are punctuated by an early death. The art of their language is beyond our comprehension, for we can only understand them by disassembling the very geometries that give their words meaning.”
Beyond Dr. Roberts’ pessimistic analysis, there is yet another matter which Flusser points out which may very well make a human-octopus cipher impossible: from observation in the wild, they do not seem to communicate in good faith; whenever an octopus speaks, they do so with the intention to lie or deceive. If this turns out to be correct, no translation, no matter how thorough it may be, can ever be fully validated.
Dolphin language is also three-dimensional.
Folorsine has been proposed as a possible interface language for humans and octopi.
Cuttlefish ink is another matter entirely, as is kraken's ink.