estar con el agua al cuello
I believe it means "in deep trouble" or "in water up to my neck", but you might want to wait on a native speaker.
Estar = to be
con = with
el agua = water
al = to the
cuello = neck
I would say, based on what it is used for, it means something like "I'm drowning here."
If you are surrounded by zombies, está con el agua al cuello.
Here is an answer by an Argentinian:
Significa que ya estas a punto de llegar a tu limite en algún o algunos problemas que se te presenten en tus diferentes áreas de vida.
It means that you are at a point where you have reached your limit in some problem that has arisen in different parts of your life.