There are many different kinds of bacteria that live in the oceans and perform very similar functions to the ones on land, so I believe that the process is pretty similar to the one described above. But in the absence or limitation of oxygen the process is called anaerobic decomposition. The byproducts of this type of decomposition include methane and carbon dioxide, and tend to create foul smelling gases. In fact, this is part of why swamps and mudflats often have a bad odor.

The same way as other animals do - except that theirshells are much more resistant.

Here, we can also turn to bacteria. In shallower waters, bacteria are in high abundance and can start to 'eat' invertebrates after they die. As you go deeper in the ocean, around 300 ft, there is not as much dissolved oxygen in the water and the pressure starts to get quite high, such that bacteria can no longer survive in those conditions.At great depths, many marine invertebrates can be preserved, intact,for quite a long time.