It rather depends which censors you're talking about. The UK censors wouldn't care, I can tell you that. But I know the US has a completely ridiculous system that treats adult audiences like children.

Anyway, I enjoyed The Shape of Water, and my wife thought it was even better than Pan's Labyrinth (I don't agree with her). I really must give it another look. Del Toro evidently has a think for fish-human romances, given a similar (albeit much more family friendly) romance in Hellboy II: The Golden Army. :)