Definition
To be left to succeed or fail by one's own efforts without any assistance.
Start-ups often have limited resources, and employees must find their own way to solve problems; it's like they are left to 'sink or swim'.
In a rigorous training environment, trainees are required to 'sink or swim' to learn and adapt quickly.
I was thrown into a brand new work environment with no guidance; I had to 'sink or swim'.