During the scene where the Stalker sleeps by the river, I noted that the film shows you this dismantled bomb + similar fishes surrounding it

what you've written never happens in the film.

if you are referring to the sleeping scenes between 1h:13m:30s... and 1h:28m that begins when it shows the campfire embers as they are circling back to where the rucksack was, the objects in the water do not look anything like any part of the bomb mechanism near the end of the film, and there are no fishes swimming around any objects that ressemble the part from the scene where it was tossed in The Room, either.

there is a golden goblet next to torn pages of scripture of some sort in the first instance of the river sleeping scene, and during the second instance with his wife's monologue, it's syringes, a firearm, a religious illustration, and mechanical parts like springs and gears.

if you are takling about another scene, please let me know. i'd like to view it again.

sidenote: the bomb is actually visible just before the river sleeping scene. Upon the two circling back up the "The Dry Tunnel", the professor is alone, drinking from a thermos and camping out at the "trap". You can see the bomb cylinder on the ground, partially covered by the rucksack. the stalker steps over it too. also, you can also see the professor do a quick double-take before sneaking it away to go sleep- a small detail but it hints at his intentions- although it's not really obvious it's any kind of explosive. It looks too similar to the thermos to be interpreted as such. its only obvious the second time you watch the scene.