1)To be out of one`s depth. -you are doing something that's beyond your understanding or competence or something's just too difficult for you to deal with.
"This math and physics stuff is totally out of my depth!".
2)To keep one`s head above water.- , to manage to cope with problems, especially financial, to barely get by, manage to make the ends meet ...
"Paul had to apply for bank loan just to keep his head above water".
3)To be on the crest of a wave.-to ride the crest of a wave - to be successful for a short period of time.
"The company was riding the crest of a wave when the gas prices went down".
4)The company went under.- the company went bankrupt.
5)To be in deep water. - to be in deep trouble.
"Marcus stole money from the cash register. He is in deep water now!"
6)To make a splash. To attract a great deal of attention.
"Feel good about yourself and make a splash by wearing the newest line of make up".
7)To test the waters.- to get acquainted with a situation, put your feelers out, check things around, etc.
Before I start looking for a job, I'd better test the waters in our job market.