https://en.wikipedia.org/wiki/Diving_reflex

The diving reflex, also known as the diving response and mammalian diving reflex, is a set of physiological responses to immersion that overrides the basic homeostatic reflexes, and is found in all air-breathing vertebrates studied to date.
[...]
The diving reflex is triggered specifically by chilling and wetting the nostrils and face while breath-holding, and is sustained via neural processing originating in the carotid chemoreceptors. The most noticeable effects are on the cardiovascular system, which displays peripheral vasoconstriction, slowed heart rate, redirection of blood to the vital organs to conserve oxygen, release of red blood cells stored in the spleen, and, in humans, heart rhythm irregularities.

Emphasis added. Since we've been able to observe it in all studied air-breathing vertebrates, it's likely (to the point of near certainty) that it comes from those "fish [ancestors]" you mention in your title. Pedants may nitpick the phrasing. Can't say whether that's less, more, or just as responsible for waking a person up vs. other environmental changes (e.g. loud noise, sudden motion, smell of smoke) that might startle a person into alertness though.

It's... Not a myth

It's a survival instinct.

Yes, it is true. The thing being triggered by the cold water on the face is known as the Mammalian Diving Response.

It's purpose isn't to wake you up exactly, but to prepare the body to use intrinsic oxygen (the oxygen already inside your body) as efficiently as possible.

The physiological changes this causes however, also wake you up.

It wakes you up because of the temperature shock... Not because of tiktallic...

That’s an easy hypothesis to test, because all land vertebrates have fish ancestors.

Do you see all land animals washing their faces with cold water to wake up?

Cold water wakes you up because its a shock, in the same way stubbing your toe would wake you up. Its not something specific to fish, but technically go far enough back and we got much of our sensory system and brain from fish (as did almost every other animal on the planet).

..Washing your face with cold water will wake you up…

"ancesters"

Ancestors*

You can make up anything about your ancestors or some kind of evolutionary adaptation. It's a free for all, we can't actually tell what's true. Maybe splashing water in your face is triggering an emdr session, who the fuck knows. But my bet is that it's just trying to induce horripilation, and it isn't something complicated.

it's not a myth. rapid temperature changes shock your nervous system.

myth is stupid crap like Germans thinking you will die of sickness from having wet hair.

baffles me people use the internet and come to the conclusion facts are myths and myths are facts.