Actually, in the answer above, crabs are actually detritus eaters, which is a type of scavenger and not a decomposer. That said, when having a salt water fish tank, it is very important to establish the bacteria in your system. They are responsible for decomposition and also for breaking down ammonia levels and converting those back to gaseous nitrogen or nitrates. Therefore, in the sea, bacteria are very important members of the marine food pyramids. The nitrates released help producers like algae to multiply which are the basis of the food chains.
Anything that eats dead plants or animals is a decomposer. In the ocean, there are many things that do this. Other examples not mentioned in the first post are some kinds of seaweeds, crabs, and various kinds of fish. Mostly, however, bacteria and fungi are the things that tend to break down detritus -- they are the main decomposers.
The number of Ocean Cleaners or Decomposers is very high, since Ocean biome is the biggest.
The decomposers are specific to each zone of the ocean. The presence or absence of the sunlight and the temperature at different depths, are the main characteristics of different zones of the oceans.
Examples of decomposers in oceans: bacteria, fungi, shrimps, worms etc.