Rivet Joint listens to comms and electronic signals and can auto-triangulate to locate their origin as well as analysing their content. Sentinel uses a radar to produce detailed ground maps or indicate moving targets on land. There is no overlap between these capabilities. Maritime patrol aircraft use a combination of the aforementioned techniques to monitor surface vessels and the periscope / communications masts of any submarines that may be exposing them, although constraints of space mean that the neither the radar nor the comms intercept kit is as powerful as its "single-role" brethren. For subsurface reconnaissance, combinations of sonobuoys (air-dropped sonar buoys which transmit signals back to the aircraft) and magnetic anomaly detection help to pinpoint submerged submarines. When you consider that most MPA need also to employ weapons like torpedos, depth-charges and anti-ship missiles, and be able to drop survival equipment like dinghies, you can see that they have to be a "jack of all trades" just to fill one role!
Nimrod was given an overland capability by fitting an electro-optical WESCAM, basically giving it something like a Predator's capability. This won't happen to P-8! The maritime radar can be made to produce a poor man's imitation of a Sentinel or JSTARS capability with some specific coding. References to using the P-8 as a replacement for Sentinel in the overland surveillance role are based around its expansion potential as shown in the concept image below, where it would carry a dedicated land surveillance radar as an aftermarket add-on.
