Deep water often contains a lot of dissolved inorganic carbon from sinking organic matter which would be released by the process, and so would greatly reduce, if not eliminate, any benefits. (newscientist.com)
[Response: Outgassing is very regionally dependent since it is a function of the upwelling of deeper (colder) water that is high in dissolved inorganic carbon (mainly from the remineralisation of falling biological matter and the advection of polar water to depth). (realclimate.org)
The reservoirs are the atmosphere, the terrestrial biosphere (which usually includes freshwater systems and non-living organic material, such as soil carbon), the oceans (which includes dissolved inorganic carbon and living and non-living marine biota), and the sediments (which includes fossil fuels). (sciencedaily.com)