^^P8i can only be used for Defense only..
often p3's and now P8s would work in a Hunter-killer role, on the lookout for subs. When a sub is known to be in certain segment of the ocean, these planes are sent there to actively search for those subs.
India can increase the effectiveness of this aircraft b orders of magnitude by implementing something similar to the American Integrated Undersea Surveillance System (IUSS). One of the basic components of such a system is the SOSUS, an acronym for Sound Surveillance System, which is a network of undersea sonar(both active and passive) and other listening devices which keep an eye out of submarines and oceangoing vessels and to triangulate their position..and even identify them(at times)
http://en.wikipedia.org/wiki/Integrated_Undersea_Surveillance_System
India is an Envious position of being having an almost "landlocked" ocean. Its bounded on the East by the SEA islands, and on the west by the African continent.
Thus the paths enemy(in the event of war) subs have to take to reach the ocean is riddled with choke-points which can be utilized to great effect by the IN.
a system like SOSUS/IUSS would give a general bearing (within a few miles at most) or even real live-updates of the movements of targets. This would result in the Poseidon's and other anti-sub assets(eg ships) being sent to the area to deal with the threat. For submarines safety lies in anonymity. And when anonymity is compromised, a sub is as good as dead.
btw, these listening stations are not prohibitively expensive(quite cheap actually) and would help the IN secure the Indian ocean very effectively.
a possible system could look like this...initially based around existing Indian bases/listening stations in the Indian Ocean and around the SEA Choke-points. Later such systems could be extended to the area within the SEA islands (albeit out of territorial waters).
usually the SOSUS not only acts as a means to locate enemy subs..but also as trip-wires...when a target crosses/comes within range of multiple sosus-lines..they also help establish the intentions of the sub, to a certain extent.
The red lines signify possible locations SOSUS-ish lines(this is, i hasten to add is only meant as a pictorial representation of my textual post above, and not based on actual scientific data). The general idea represented on the image above is the coverage of the choke-points and secondary and tertiary lines