You have to understand that during times of war in hostile areas there were always crew men looking out for dropped depth charges it would be very 'careless' for any captain not to make some effort to spot them to protect his ship and crew from potential slaughter that could be avoided.
Back during ww2 radar was not that effective at detecting object and ships ( had trouble in telling what it was) and the sound sonar technology was very basic, any charge detected on a radar could easily be mistaken as a large 'fish' (i use that word loosely)
Part of the look outs job was not only to spot enemy ships, but also to look for charges in the water and other dangers, such as rocks in shallow uncharted water.
This is well documented facts that you can find in any museum, or historical record!!
Even today with our modern technology there are still 'spotters' to look out for dangers on the water its been common practice since the dawn of sea faring travel and more so at the time of war.
I have genuine reasons for pointing out your comment as an error in judgement as I have a sea faring family (grandparents one served in the navy, the other in the air force, my uncles served in the army and my brothers army and navy) my brothers still serve today, and am am told first hand of practises that go on, as well my my own knowledge read from books, and my general interest in navel combat history.
The idea of dropping charges in the water is to sink enemy ships/boats/subs or to stop them crossing into areas controlled by other parties , this saves the amount of vessels needed to patrol certain areas constantly where as smaller crafts that require less resources to function can patrol the 'mined' areas, (8 men crew or up to a few 1000, which is more expendable during war?!) this helps maintain a strong fleet that can deployed to areas there needed without compromising areas they secure, as they are still protected in some form.
The other main factors of depth charges is true that they are deployed into paths of ships, since in some waters they have limited manuvarability, and can only travel in certain charted paths they become easy targets for fast drop deployments, this is what you are referring.
This is common practise for war time, but it is one of many practises, and is most spoken off simply because its the most 'heroic' acts where as it is also the most dangerous. (The best stories always live longer.)
Its similar practise to the torpedo launch boats, fast attack crafts that can drop powerful weapon's into a target and escape before any damage can be caused to them, or to fast to be tracked by big slow turning deck guns.
These are all practise of modern warfare but be sure not to over look that fact that dropping charges is more a tactical defence rather then an attack strategy, just this practise has the ability to be both at the same time.