Biomes are generally defined as a vast areas of ecosystems connected together to form communities located under similar climate or vegetation. Usually it is defined as "diverse groups of ecosystems".
In starting an informative essay about ocean biome, first you have to define what a biome is. The different kinds of biomes existing on earth (aquatic, dessert, tundra, grassland, forests...) and how they differ with each other. The next thing you have to emphasize is how ocean biomes differ than other types of biomes. Having said that, you have to get a clear picture of how the ecosystems work in those areas.
There are vast sources that you can select. Wikipedia has a strong description of biomes. Look at the reference section so you can be able to see the authors of the sources. National geographic also has a good description of aquatic life which can be useful in your research.