Daenerys has Varys on her side who is some kind of master spy.  

She also has Bran who can scout for her easily.  

She was up in the sky from where she should be able to see Euron's fleet where they were hiding before she was in range of the scorpions (if the ships could shoot at her, she should be seeing them as well).  

She has a strategist Tyrion who knows about the existence of this dragon-killing weapon when Bronn attacked her during Battle of the Gold Road.

How is it believable that Euron could ambush Daenerys so easily?