How do you know the lake is real?

I've seen it. It could have been a very convincing fake, but.. if that doesn't count, then nothing does.

How do I know you're not in on the conspiracy.

Maybe it was just an oversized puddle, and not actually a lake.

The fake water there is very cold.

Amazingly no, the lake is called Ness, Loch just means "lake" in the old regional dialect.

And Frankenstein was the name of the doctor.

You can think of Frankenstein's monster as the doctor's son, so they would also be a Frankenstein.

Ohh nice one :)