Several things come to mind immediately.
First is that "depleted" uranium is a very good neutron shield. Not even lead is as good as U-238. And the toxicity of uranium is far above arsenic, mercury, lead, iron, tungsten, and titanium. There is probably a fraction of a millimeter of uranium in that shielding. The plastic will serve to stop alpha particles from said uranium, but the lead would do the same. However, the uranium would be most efficient if it was furthest from the reactor, meaning that the lead would be closer. That plastic probably shields some steel, which is shields the uranium, which shields the reactor.
Second is that plastic will prevent galvanic corrosion and surface oxidation in that oxygen-enriched area. Zero maintenance is good maintenance on a boat. Anything you don't have to paint on a regular basis is, like, totally cool.
Third is that, if there really was a thermal event, that plastic would melt. Possibly sealing off the room. Yes, that plastic will breakdown and form vapors, but every last little bit of shielding and/or prevention is taken into account when a boat is designed. The engineers get paid to do their jobs, and given all considerations (both political and resource), boats are actually designed well. Things that sound stupid might very well be up until that one lucky break where they are a saving grace.
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