Color me confused. I've seen several articles on how the Navy has selected the Boeing 737 to be its replacement for the Lockheed P-3 Orion as a submarine hunting vehicle (the best of which is here). The 737 does not compare favorably to the existing P-3 for this mission.
The P-3 originated from the Lockheed Constellation airliner back at the beginning of the jet age in the 1950's. The Constellation was eclipsed by the Boeing 707 and its ilk which were capable of much better performance. It found a niche use in the Navy as sub-hunter and maritime reconnaisance vehicle, for which its slow speed was an advantage, not an impediment. The 737 has been one of the most successful passenger jets of all time, with Boeing having sold many thousands. It was designed for much higher speeds, though its size and range are roughly comparable to the P-3.
Submarine hunting requires the ability for long endurance, low and slow flying. The P-3 handles this mission beautifully. According to this site, it has an endurance of 16 hours at 203 knots (233 mph) and an altitude of 1,000 ft. In contrast, this site lists the 737 specifications. It has a much higher cruise speed (533 mph, and presumably at a much higher altitude, though that's not listed), and a much reduced loiter capability. The site does not list its loiter ability but calculating based off the given cruise speed (938 km/hr) and the cruise range (6,038 km) yields an endurance of 6.4 hours. It is possible that flying slower could improve this, but not by enough to match the P-3. And this is also at a higher altitude at which the vehicle will not likely be operating.
The P-3 is able to loiter more effectively because it is a turbo-prop powered vehicle (see this old post of mine on propulsion systems). Turbo-props are more efficient at moderate subsonic flight speeds (about half the speed of sound). It does not make sense to use a turbo-fan powered vehicle like the 737 for this mission. It makes even less sense to use a vehicle with only two engines (note that the P-3 has four). A submarine hunter will spend much time over the water (obviously) and, even with today's much more reliable jet engines, it still makes sense to have some redundancy for this type of mission.
The only explanation I can come up with for how this vehicle won is that the decision was taken for purely political reasons. The Strategy Page article I link to above hints at this, saying the decision may have been influenced by Boeing's losing the Joint Strike Fighter contract. The contract is also a cost-plus contract, where Boeing's profits increase the more expensive the vehicle is, creating all kinds of dis-incentives for efficiency.