The Tsar Fish has been stated to attack heretical individuals who make use of electricity while leaving the technology-free worshipers unharmed; but will freely attack anything in the water. This is easy enough to explain; water is more conductive than air and so electrical fields generated by living organisms (primarily by the activity of the nervous system) can be readily sensed whilst submerged.
The reason it attacks Artyom and other heretics when they are above water is that it can sense the significantly stronger electrical fields produced by their technology even when they are not submerged.
Though it does seem to possess sight and hearing, those senses may be better suited to navigation than hunting.
One final detail of note: both Sturgeon and Catfish (the two likely candidates for the Tsar Fish's pre-mutation species) are known to possess Electroreception. Sturgeon in the form of ampullae of Lorenzini and Catfish via sensory pit organs.
