#1 – A Fish Out Of Water
= feel uncomfortable, like you don't belong in a particular situation
I don't have any experience working with kids, so I felt like a fish out of water when I had to teach a class of 6-year-olds.
#2 – Come Hell Or High Water
= if you say you will do something come hell or high water, it means you will definitely do it, no matter what difficulties appear
I am going to run a marathon this year, come hell or high water!
Warning: the word "hell" can be offensive, and in professional situations it's best to use the expression "no matter what" instead.
#3 – In Deep Water
= in a difficult situation – especially one that is beyond the level of your abilities – or in trouble
A number of customers have taken legal action against our company – we're really in deep water now.