By RAW, is the Giant Rocktopus from the Out of the Abyss adventure unable to breathe while out of the water?

Question

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?

Answer 1

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.

This creature is a giant octopus that has evolved to live and thrive on land

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.

Answer 2

Rules as Written is initially unclear.

The rules as written explicitly states that a Giant Rocktopus can live and thrive on land.
The rules as written also implicitly claims that a Giant Rocktopus cannot breathe on land, and can only spend 1 minute (CON 13) at a time outside of water.

These are contradictory statements.

The best way to resolve this contradiction is to apply the 5e principle of specific-beats-general:
The specific rules text about a Giant Rocktopus is correct over the general text of this variant's base creature.

As such, the Giant Rocktopus is able to live and thrive on land, and can breathe air.

With these considerations, Rules as Written confirm that a Giant Rocktopus can live outside of water.