King Canute...
...is generally misunderstood.
He commanded the tide to stop to prove that he *wasn't* omnipotent, knowing it would fail, to chastise some overly sycophantic courtiers. In other words, he wasn't the deluded one. It's a pity popular myth has cast him in the exact opposite role.
/There's a plaque on Canute Road in Southampton that reads 'Near this spot, Kind Canute reproved his courtiers'.
//On reflection, anyone prepared to get contaminated by the waters of the Solent must be mad...