up a creek without a paddle

To be up a creek without a paddle means you are in a situation with little to no chances of being rescued or getting help. This is another way of saying you are in trouble and in a very difficult situation with practically no way to get out. By saying you are up a creek without a paddle, it refers to literally being up a creek with no paddle which in this situation would be the only possible way to help you unless you got out and tried to swim to safety.