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
My dog can’t feed herself. My family seems to like having me around.
Get off of social media. Other than here I really keep my news to computer stuff that interests me