I wouldn't call that a fuck up by any measure. This seems meaningfully distinct than just "difficult terrain", since its a hazard in itself. It's not a matter of just going slow, just staying there is dangerous. Not to mention it can compound with difficult terrain.

In practice, I'd still prefer the 5e rule where everyone has at least some time they can manage being in there as opposed to just rolling con and having your wizard drown immediately when they touch water like it is a video game.