As mentioned in other answers, the size of the preator is largely determined by the size and availability of the prey. If the oceans are full of pods of whales, then an extremely large predator would become viable, since there would be a reliable supply of food.
The other issue to look at is "why" a predator has to evolve to such a size. On land, mega predators arise wherever there are megafauna, such as the last ice age. Megafauna are advantaged in that environment since the square-cube law means that they have less surface area relative to their size to radiate heat, so a mammoth is much less likely to suffer hypothermia than a lemming. Since the prey animals are so large, a much larger and more agressive predator is needed to successfully hunt them, such as dire wolves, sabre tooth tigers and short faced bears.
Sabre tooth tigers suggest another thing in this ecosystem. Large prey animals might need different methods to attack and kill. Biting and holding the neck to choke the prey works for a tiger to kill an antelope, but would be totally ineffective for a mammoth. Sabre tooth tigers evolved long stabbing canines to deliver deep puncture wounds that caused prey animals to bleed out. A possible analogue in your wourld would be a narwhal, although the modified tooth in that case is used to lift molousks from the sea bed. The swordfish suggests an alternative to deliver slashing blows to prey, weakening it to the point that the jaws and mouth can come into play. Other methods might suggest themselves as well.
So imagining megapredators really involves thinking about how the entiire ecosystem works. why do such large prey animals exist, and how do the predators deal with them. Other questions like scavengers and parasites should also come to mind in a fully developed ecosystem.