Is there a maximum size an ocean bound creature could grow to?

It could not grow big enough to exceed its environment. It would have to be smaller than whatever ocean it lived in.

Its size will be checked by natural selection. If the animal’s size is exceeding the potential energy input by the available food sources, the animal will die before it can breed.

Blue whales are the largest thing that has ever lived on Earth. They eat krill.

What a great question! Let’s think about this a moment.
All living things are complex chemistry experiments that follow the rules of thermodynamics. To function and thrive they need basic chemicals, nutrients, and a way to eliminate waste products.
If the creature is plant based it can grab CO2 and little N2 from the air, get trace minerals from the water and use sunlight to make what it needs. In that case the creature size would be limited by the surface area of the water.
If the creature is “an animal” it must get its nutrients by eating plants or other animals. It needs a food source to survive and the food source must be big enough to sustain the creature’s daily needs indefinitely. For example, let’s say a plant will grow 5% per day under ideal conditions. At equilibrium, the sea creature can grow until its daily caloric needs matches the growth rate of the plants (5%). So it needs a supply of plants in its immediate area that are 20 times more plentiful than its daily meal.

If the creature can travel it must not expend more energy that it gets by eating. Food and nutrients are the limiting factors.
The creature would also need a way to reproduce, either sexually or asexually. The food supply would need to be big enough to sustain its progeny.

Maybe we can invent a super creature that covers all bases like a Euglena.

If it is “bound” by the ocean theoretically a single organism could encompass the entire ocean, essentially replacing it. It can be argued that the ocean itself is an organism depending how you stratify organisms. Humans are a collection of organisms yet we are considered one entity. It depends on your perspective.