do fish drown?

Yes they can in a sense, you could argue that taking them out of water means that they drown in air since they are in an unusual medium (air, not water) and cannot breathe properly as a result. Fish can also drown in water (though you should probably call this suffocation) if their gills are held shut. This does happen occasioanlly when soemthing gets stuck on the gills and is known to occur when frogs in the breeding season mistake fish for a female and hold on to them, closing the gills and stopping the fish from breathing proeprly till it 'drowns' underwater.

Similarly pollution in a river which depletes the water of oxygen could also be termed "drowning".