Oh hell yeah!

Neat thank you!!

And I learned a new word

Zeitgebers are external cues, such as light, that synchronize the circadian clock in organisms with the Earth’s day-night cycles.

Mammals have to breathe, so they are surfacing from time to time… so roughly they’ll know day from night.

Tides are mostly noticeable near land.

So you have deep sea creatures away from shore. They mostly won’t care about day and night as there’s no difference, with light not penetrating more that about 200 feet.

So below 200 feet, and away from shore you mostly can’t. But also it would not matter as there’s no difference

But if you were a human, in a submarine or a diving vessel where you needed to know, a clock or radio… would be about the options