I think we can agree on....
Mark meant relatively near the surface. What exactly that means has yet to be determined.
The experts in the subject are far from agreement on a precise interpretation of the data.
Although NH3 + H20 is conductive, spectroscopy of the Europan suface reveals that the content of an ocean is mostly MgSO4 (assuming it exists).
For a free, quick, and relatively recent intro to the current state of thinking on the matter, see the abstracts from the Europa workshop at LPI from Feb '04.
