(from http://www.business-english.com/swimmingidioms/menu.php with some modifications)

If you are ‘out of your depth’, you don’t have the necessary knowledge, experience or skill to deal with a particular situation or subject. In North America, a common replacement is ‘out of your league’, as in major league baseball.
• When she started talking about quantum physics, I felt completely out of my depth/league.
• I’m an engineer. I feel out of my depth when we discuss accounting problems.
• That woman is so beautiful. She is definitely out of my league!

If you are on ‘the crest of a wave’, you are being extremely successful or popular. If something is popular, you can try to ‘ride (on) the wave’.
• That singer is on the crest of the wave in the pop charts at the moment. You can hear his music everywhere.
• He became successful riding on the wave of using British actors as villains in Hollywood movies.

If you don’t get any training before you start a job or activity, you are ‘thrown in at the deep end’.
• Everyone was off sick so I was thrown in at the deep end.
• The best way to learn the job is to be thrown in at the deep end.

If you are struggling to spend less than you earn, you are trying to ‘keep your head above water’.
• Since they increased my rent, I’ve been struggling to keep my head above water.
• With the new sponsorship, the team should be able to keep its head above water.

If a company has to stop business because of losses, it ‘goes under’.
• The company couldn’t afford to pay its suppliers and it went under.
• In this economic climate, a lot of businesses will go under.

If you are in a very difficult situation, you are ‘in deep water’.
• If the bank doesn’t give us this loan, we could be in deep water.
• He was caught stealing from his company and now he’s in deep water.
• Note: this has been commonly replaced with the more street-slang phrase ‘in deep sh_t’. This of course is a curse-word.