It's not pressure, it's water temperature. The above link is to an oil tanker that went down after a collision off San Francisco Bay in 1953 while heading to Korea with war supplies. This wreck's position was not known with any certainty until recent times, but mysterious spills after storms reminded people in the area that it was out there.
The vessel is only in 180 FSW, but the fuel has to be heated in order to pump out the holds -- which was done to partially remediate the risks associated with a future major hull breach.
If the airliner "soft landed" (extremely unlikely) it is barely conceivable that the plane went down with its fuel tanks intact. (Even a controlled landing in open ocean is likely to break up the aircraft).
The point that I'm trying to make is that this plane, if it's there, is in 19,000 FSW. That is in excess of 3 miles below the surface. We are talking very cold water. I would expect even fairly light Jet-A to become very gooey at that depth.
But even if it didn't then this fuel has a long way to go before reaching the surface. There are a lot of sub surface currents & temperature gradients to cross. Any slick at the surface is bound to be so dilute as to escape notice as "a slick".
OTOH, I suspect that the aircraft either ran out of fuel or lost the ability of controlled flight and went in hard. In that case what little fuel the aircraft had has long had a chance to dissipate.
If that's the case and the SAR experts know that and the oil on the surface can't possible be from the plane, crashed or submerged, then why are these experts taking note of this oil slick and sending 2 liters of it it in for identification???