A few limits more
For creatures subject to the limits of size imposed by the square-cube law as described in Tim B's answer there is no point in considering the problems met at still larger sizes. So for whales, squid and fish, for example, see that answer. However, for creatures of less conventional shapes, size can be extended further, to the point that it may be worth considering the further limits that are met by hypothetical creatures of truly huge size. As we progress through these limits we get to life forms where the definition of "creature" blurs a little, so your own requirements of what should constitute a creature will affect how far you can go in your own world.
Outgrowing the square-cube law
For a consistent body shape doubling the length quadruples the surface area but octuples the volume. If the internal distribution also stays consistent this also octuples the mass, making maneuvering more and more difficult. Buoyancy only helps with the support problem of mass. The inertia of a massive creature still restricts its movement, and as Tim B points out, the available surface area for propulsion cannot keep up with the much faster growing mass as a creature is scaled up.
The restrictions of diminishing surface area per volume can be overcome by changing shape during growth. The problem only applies to a creature that keeps the same shape as it grows. A creature that constantly increases the relative surface area of its shape as it grows would be able to maintain contact with a large surface for food and oxygen intake, as well as propulsion.
For example, animals that exchange oxygen and carbon dioxide through their skin are much smaller than animals that use gills or lungs to change their shape and increase their surface area. Even scaling up a lung does not prevent the surface area lagging behind the volume - a larger creature has more levels of branching in its lungs, rather than just the same shape scaled up. For example, the number of levels of branching in a mouse lung, if scaled up to the size of a blue whale lung, would have a far smaller surface area than the lung of a blue whale. A large lung is not simply a scaled up small lung - it is a closer approximation of the fractal tree structure of an idealised lung.
So the gas exchange problem can be fixed with a self similar structure that is not simply scaled up but grows more complex at larger sizes. However, this still leaves propulsion limited by the square-cube law. Even though lungs make possible creatures considerably larger than a blue whale (the largest reported blue whales are almost twice the size of the average blue whale), propulsion means the square-cube law still applies eventually.
Note that blue whales can move considerably faster than the speed they use for feeding - their size is not the upper limit even based on propulsion. In the absence of predators (orcas) and the need to migrate large distances between feeding locations and breeding locations, blue whales could feasibly be considerably larger and less mobile.
Externally (branched body)
To grow beyond the limit imposed by propulsion a creature could have an external body structure that is branching, rather than just branching internally. This would allow most of it's body to be surface, available for gas exchange, feeding and propulsion. Not only would more area be available for movement, but movement may be less important with access to so much more water in contact with its surface. Feeding through its entire surface (like the trichoplax) rather than only through its mouth, it may not need to move very quickly.
Limited by pressure
The depth a creature can descend to is limited by the pressure it can withstand. In a similar way, the size of a creature is limited by the pressure it can withstand, since a deeper/taller creature will have its lower body at higher pressure even when its upper body is at the surface.
Even if the creature's outer surface can withstand the pressure, its internal workings will have difficulty beyond a certain depth. Lungs cannot be inflated against immense pressure even if the rest of the creature can stand the pressure. The heart will need to be stronger to maintain the required very high blood pressure, and the entire circulatory system will be subjected to this pressure, even in those parts of the animal in lower pressure water.
Growing beyond this limit could be done by growing laterally, leading to a wide shallow creature rather than growing deeper. Alternatively having no circulatory system would allow a creature to grow deeper. However, this would require a much more branched body structure with very little internal volume - most of the body being near the skin. This can still be seen as an animal but at first glance the shape is beginning to look more like a motile plant or fungus. Somewhere in between you might find creatures that have no global circulatory system, but several local circulatory systems each restricted to a given pressure range.
Changing to a different fluid (for example a methane ocean) doesn't remove the pressure problem. Different liquids are compressible to different extents, but even with very little compression the pressure still increases with depth - you can't get around that by changing the liquid. A less dense liquid will see lower pressure at a given depth, but there will still eventually be a depth at which pressure is too great.
Limited by available water
This is not just the water required to accommodate the creature, but the water required to accommodate its food supply (whether plant or animal or microbial). With the depth limited either by pressure or by the total depth of the water, the amount of available water depends on the surface area of the body of water.
Limited by available energy
If the energy source for the ecosystem is sunlight, then only a volume near the surface is taking in energy. Even in the most efficient case where there is no food chain and the creature takes in sunlight directly (some animals use photosynthesis) this restricts the size of the creature to a limited depth. It can be the same size as the surface of the water it lives in, but it can't extend arbitrarily deep with a limited energy supply.
Limited by available raw materials
Just having enough water and energy doesn't allow for growth. A creature needs the raw materials from which to build its body. Whether these are absorbed directly from the water or obtained from food, ultimately those materials come from the water. An ocean of almost pure water with very few dissolved materials will support very little life, limiting the number and size of creatures. Even a single raw material being limited can have this effect. For example, growth in Earth's oceans is mostly limited by the shortage of iron. There are enough raw materials for an explosion of growth but those raw materials cannot be used at that rate because there is insufficient iron. Dumping a source of iron in the sea leads to algal bloom - a sudden increase in the growth rate.
Limited by planet size
With depth limited by pressure, the habitable size of an ocean is limited by the surface area of the planet. A given size planet can only have a limited ocean area, even if the ocean covers its entire surface. Simply making the planet larger only helps up to a point. Beyond a certain size, a larger planet does not provide more water volume due to the constraints of gravity.
Limited by gravity
The larger the planet size, the higher the gravity at the surface. This means that the water pressure increases more quickly with depth. Not only does this further restrict the depth that a creature can occupy without being crushed, but even a creature that can withstand great pressure will be restricted by the fact that higher gravity means shallower water. The higher the gravity, the less the depth of water before the pressure prevents it being liquid. The ice we are familiar with is less dense than water and floats on top of it. Subjecting this ice to intense pressure makes it melt. However, subjecting liquid water to still more pressure forces it to become a different type of ice that is more dense than water. There are several types of high density ice, but the relevant point is that below the depth required for forming this ice, there will be no liquid water. Increasing the temperature of the water can increase the depth at which this happens, but for sufficient gravity there will still be an ice boundary that limits the depth, and you can only increase the temperature so much before you are just boiling off the surface water and defeating the object.
So yes, there is a maximum size limit to an ocean bound creature, but just what that limit is depends on what you count as a "creature" and how extreme an environment it can survive.