Lego survives being eaten by babies and poop back out again (source: my younger brother). They also survive being left out at theme parks (ie the various Legolands) and primary schools under all weather conditions for months and years.

So I don't think I'm underestimating the resilience of Lego bricks to flooding.

There was an article a on HN a while back about the plastics chosen by Lego. They put an exceptional amount of time and effort into choosing durable materials for their bricks.