In real life, diel vertical migration is well-attested in marine animals and has various explanations (including following zooplankton, and predator avoidance by only going up to towards the surface when visability is low). To my knowledge, this type of behaviour is not as easy to observe in apex predators, but it follows that they can theoretically modulate their behaviour to follow their prey to where they concentrate -- even if visability isn't much better close to the surface compared to the deep during the night.

As for your heat comment, I've seen passing references to "hunt warm, rest cold" as an argument for a form of vertical movement in ectothermic predators, but it has more to do with movement between shallow and deep, rather than day and night per se. After all, sharks have a different way of regulating body heat (and thus conserving energy) compared to lions and bears.

Exceptionally excellent low light vision. Sucks to be in bright light, you're blind.

Squid are relatively nocturnal and could avoid daylight because their eyes are so large/optimized for low light conditions.

Well the answer that comes immediately to mind is: that's when its food comes out.

Being on the surface during the day means surface predators like birds eat you. So many animals only become active there at night. But that's when the apex shows up. Drawn by all the activity. It is less frequent than the birds, so dealing with it is the preference. But it is still a fierce killer

Octopus, they are very intelligent so its sneaky tactics of hiding in areas of low lighting make it a very dangerous predator. Give it bioluminescence at its tentacle tips and it has a very good reason to stay in the dark it can draw in , prey, ships unsuspecting fisherman in. Depending on its size the bioluminescent patterns can be made to look like torches in the night. Playing on its intelligence it could pretend to be a light house guiding ships away from the shore and back out into the open ocean where it can more easily take it down. One trait of bioluminous Creatures is their adverseness to light. This octopus could have massive eyeballs that are very sensitive to light. This could prove to be a way to deal with them if they are encountered but also a terrifying one. After all octopus are very intelligent and it’s very unlikely that the same blinding trick will work twice.

Leftover behavior from before it was the apex

Maybe an albino predator? It's skin doesn't handle direct sunlight well so it slumbers when the light is out and only comes out after sunset.

hunts the surface at night because thats when the prey is vulnerable? It could eat resting sea birds or fish that feed during sunset or hunt around algae columns that are too difficult to access/hide in during the day. You could also have no reason, thats just how the animal is?

Keep it simple. The deep is safe or absolutely necessary for the apex predator. The "best" food source is mostly only available near the surface at night. 

Maybe a (or some group of) marine mammals sleeps on the surface at night. Dolphins and sea otters and things like that, or some fantasy version. 

Bad light vision could be a reason.

Was there an extinction event in your world? Perhaps the ancestors of this species migrated to the depths to feed on the corpses of those dying on the surface when they sunk down, and while the species now hunts on the surface, it’s a leftover trait.

You do not need to explain the motivations of your apex marine predator.

Describing the known behavior of your apex marine predator is enough.

Your characters can make any guess they want regarding the motivations. Will they ever really know for certain? People have been studying whales, which are plentiful and easy to find, for centuries and don't understand 100% of their behaviors.

You are over-thinking this. You don't have to explain the motivation of anyone except your main character(s).

Good Luck with your project.

Heat is still an issue in water. Marine life can be very sensitive to temperature changes, and your deep-dwelling predator could be highly sensitive to warmth.