Water at the bottom of the ocean is being compressed by the water above it. That compression is forcing the atoms of the water closer together. You can find more information here: http://www.physlink.com/Education/AskExperts/ae15.cfm?CFID=25001951&CFTOKEN=bd8d83931c8727f5-6410F281-15C5-EE01-B98A679D414A1B01
