A fish out of water
Someone who is in a situation they are unsuited to.
This metaphor is quite old. Chaucer used a version of it in The Canterbury Tales: Prologue:
...a monk, when he is cloisterless;
Is like to a fish that is waterless
The earliest reference that I can find to the present day wording of the phrase is in Samuel Purchas's Pilgrimage, 1613:
"The Arabians out of the desarts are as Fishes out of the Water."