The description of the Giant "Rocktopus" random creature encounter in the Out of the Abyss adventure (p. 28) provides the following details:
This creature is a giant octopus that has evolved to live and thrive
on land. It can alter its coloration to appear as a rock formation,
and it tends to lurk in crevices and fissures, attacking smaller
creatures that wander near. It has a walking speed of 20 feet and a
climbing speed of 10 feet, loses its Hold Breath feature, and replaces
its Underwater Camouflage feature with the following feature:Camouflage. The octopus has advantage on Dexterity (Stealth) checks.
The giant octopus (MM, p. 326) has a trait called Water Breathing:
Water Breathing. The octopus can breathe only underwater.
The giant rocktopus entry makes no mention of alterations to the Water Breathing trait, only to the Hold Breath trait. Does this creature still require an aquatic environment to survive, according to RAW?
Best Answer
I see where this is confusing, but I think that the first part of the Rocktopus' is probably the most important to note given the exact phrasing of your question.
Since it says that the Rocktopus has evolved to not just live, but thrive, we can assume that the answer to the question "Does it still need the aquatic environment to survive" is no, especially given that the water breathing trait specifically says that it can only breathe underwater.
However, according to RAW in the book, it seems like RAW says that it does. So, in the end, like most things, it just comes down to the DM's verdict, as this just seems like a case of Rules as Intended vs. Rules as Written.