The only thing that stopped them? Thomas Pierson Frank and the Thames Flood Prevention Emergency Repairs Team.
When Frank and his Dam(un)busters received a report that an embankment had been damaged, they would immediately head out and repair it before things escalated. And It wasn't just the threat of floods that the team had to contend with. If the public was aware of the danger, they would've understandably freaked out and started panicking, which could have led to the Nazis learning about this weakness and giving London history's worst pool party.
For this reason, the team worked at night. But you know who else was also working at night, don't you?
Despite records showing that the embankments were damaged 120+ times during the war, the actions of Frank and the TFPERT meant that they never broke.