The P-8A will use a new hydrocarbon sensor to detect fuel vapors from diesel submarines and other conventionally powered ships. No snorkeling allowed!
This is apparently what the USN feels will be better than MADS... since the USN is focused on "littoral ASW", there are too many shallow-water magnetic anomalies (wrecks, ferrous rocks, etc) for MAD to be "reliable" in finding subs. The old "go back and see if it moves" bit is less effective with non-nucs anyway, since many navies operating in shallow waters like to sit on the bottom and listen/wait, so they might not move for a long time.
The P-8 does seem to be capable of dropping sonobuoy at fairly low altitude... enlarge the pic in this story, the smoke markers in the water seem not too far below the aircraft. P-8 also carries 50% more sonobuoys than the P-3 did.
U.S. Navy Boeing P-8A Poseidon launches first sonobuoys | Boeing and Aerospace News - seattlepi.com