Um...what about the P-8 Poseidon? How does it scan and detect enemy submarines? I'm very curious about that thing.
Using its suite of sensors; Radar, Sonobuoys fitted with Hydrophones and MAD (Magnetic Anomaly Detector) mainly. There are some add-ons that are used to detect Engine exhaust fumes (for snorting subs), oil traces in water and even surface temperature anomalies.
Also, do ASW choppers have to coordinate with its host vessel to do the job?
Yes, They do, just to increase the kill-probability; through interfacing sensors and overlapping weapon deployment. Though the ASW Helo can operate autonomously as well, but with the risk of a lower kill-probability.