The pathway for the bacteria to arrive at the organic matter is far more fluid (no pun intended) underwater (marine or fresh). Matter that can sit outside of water for weeks, months, even years (or hundreds) will decompose at a much higher rate under the water.
To visually see the concept in action take a piece of lunchmeat ham. Rip this piece of ham in half. Place one half in a cup of water (marine or fresh, your choice). In one-two weeks the piece of meat in the water will be decomposing and the piece of meat that was exposed to the air will have turned hard but will have hardly (if at all) decomposed.