'a drop in the ocean', means that something will have as much impact as adding just one drop of water to the ocean. It doesn't mean that something is impossible to do, just that the doing of it is unlikely to have much impact. We say 'needle in a haystack' when we mean that the search for something is almost impossible - that finding the object in question is the equivalent to finding a needle in a haystack.

In English we say "a needle in a haystack."

Very hard to get something done is like drawing blood from a stone.

As Guo42 says, we have the expression 'needle in a haystack' - but it has a more restricted use. If something is impossible to FIND, we say that it's 'like looking for a needle in a haystack'. We don't use it for the more general meaning of something that's very difficult to do.