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[Tharkon]

I wouldn't allow aquatic familiars in non-aquatic campaigns, and not the other way around either.

Imagine a kuo-toa with a raven familiar...

An octopus on land is like a human underwater, it can move around but it can't breathe. Use normal rules for suffocations/drowning (basically the same rule despite each having their own section in the DMG).
It can stay out of water for 22 rounds initially then make increasingly difficult Con checks starting at 10, if it fails it will go into unconscious, dying and dead stages in the next rounds.

 

[jefgorbach]

thread necromancy but this was just posted to facebook today proving octopi CAN walk on land ... so presumably the aquarium myths of their extended walks are likewise true.
Octopus Walks on Land at Fitzgerald Marine Reserve - YouTube

 

[Dannyalcatraz]

Oh yeah, its no myth. In fact, I was just discussing this with another aquarium worker who talked about how some can actually run/hop over short distances rather like a small mammal...

Except they can stick to the underside of furniture while being chased...