In over your head
Definition 1 of 1
Pro Tip 1/3Figurative Usage
The phrase doesn't relate to actual swimming but is used to describe feeling overwhelmed in any scenario. 
After taking on too much work, she felt in over her head.
Pro Tip 2/3Not Physical
It is not about physical position but a metaphor for being in a challenging situation. 
Managing three projects at once, he found himself in over his head.
Pro Tip 3/3Emotionally Loaded
This phrase often implies stress or anxiety from being unable to cope with a situation. 
She volunteered to lead the team, now she's in over her head and scared.