I don’t think it’s plausible. That’s a heavily surveillanced area too, it’s not just one camera missing him but multiple. Also they’ve gone through recordings so many times.Here's one: security camera was shit and missed him leaving. He then falls into the river accidentally.
Also, it’s very rare that people drown and their body does not come up on the surface of water. We have 5-6 drownings of drunk men in Kraków every year and they are all found within two months.