Ocean depths are quite cold. The sediment build up needs to get quite thick to get the temperatures raised enough to start Thermal Depolymerization.
Yes, and I'm sure that's no problem as there must be hundreds of tons of dead plankton, algae and sea-life that falls to the bottom of every square mile of ocean floor, every day.
Additionally, it's not the same everywhere in the depths, just as it isn't on dry land, there are areas of high geothermal activity...and I'm sure areas go active and dormant over time...leaving large deposits of "cooked" compost.