Planning to buy a used car, but my friends told me not to because I might end up with a flooded one.

So I told them I'm bringing a mechanic with me, but they hesitated that some flooded cars are impossible to identify even with a certified/professional/licensed car mechanic...

Is that true? but also, one friend told me that flooded cars are easily spotted on by experts. I don't know which should I believe?

Please help me.