You may believe that the platypus is just an oddity, with all sorts of strange bells and whistles for appendages that really don't amount to much. Its funny-looking walking and swimming could lead you to think that platypuses are harmless, maybe cute and cuddly, despite minor worries about its poisonous heel spike. In the Animal Kingdom, this animal is often considered a jester. Well, the fact of the matter is that the platypus is a well-oiled amphibious killing machine.
The duck bill does, indeed, contain electroreceptors, oriented in stripes lengthwise. These receptors, somewhat similar to those found in the glass knifefish, are like pockets of gel with specially adapted neurons, optimized to pick up electromagnetic disturbances in its surrounding field. Between those stripes are stripes of mechanoreceptors -- not unknown to the rest of the animal kingdom. Imagine a long tube with one end open, and at the base, a swinging pole, surrounded by more neurons. These neurons fire depending on how the medium pushes or swings this pole around the tube. On land neither of these of receptors are especially useful; electromagnetic impulses don't carry all that well in air, and unless its prey causes a fairly large gust of wind, those mechanical sensors won't be all that useful. So while the platypus is on land, it keeps these warmed up and moist with mucus. Once the platypus gets hungry, it hits the water, a medium much more conducive to the platypus' brand of prey detection.
Once in the water, the platypus shuts its eyes and ears, leaving only the bill sense, as it is called, to locate prey. That should be an indication of just how powerful this sense is. The platypus integrates the mechanical disturbances provided by swimming shrimp and insects and the small electromagnetic impulses emitted by muscular movement. Keep in mind that these signals arrive at different times -- thus, once both bits of information reach the platypus' brain, a very detailed and accurate representation of the living things in the environment is obtained, something like a three-dimensional radar grid. Studies of platypus brains indicate that cortical real estate is divided into columns which correspond topographically to areas of the bill -- that is, that a cortical map of the bill exists. This kind of organization means that pieces of electromagnetic and mechanical information are stored in close proximity... and that the integration of stimuli from these two modalities happens very very fast.
In other words... it knows where you live.
The platypus sways its head in sort of a figure-eight pattern as it swims in a patrol phase, simply looking for anything out of the ordinary. Once some disturbance is found, the platypus engages in active search, slowing down and looking more thoroughly, sort of like a helicopter shining a spotlight down on an escaped felon; and once it's close enough, attacks furiously and with grim accuracy. The bill sense is powerful enough to detect drained batteries in the water, and more than sufficient for picking up cues from shrimp, insect larvae and the rest of its prey.
This method of predation probably explains some of the other quirks of the platypus' morphology. Its hardish fur hugs its body and, besides protecting it from cold, insulates its electromagnetic field from the bill sense, enhancing the electromagnetic signal to noise ratio. The electromagnetic field is also attentuated a bit by the platypus' tail, which works alongside the webbed feet to handle the awkward swimming that is so key to its killing power.
shrimp beware
For more information (some already used here): Pettigrew, J. D. (1999). Electroreception in monotremes. Journal of Experimental Biology, 202, 1447-1454.