pissing in the ocean
a futile attempt; an act which will have very little or no consequence; a very small proportion of the whole
Trying to stop illegal downloading by arresting individual college students is like pissing in the ocean - there is no way they can arrest millions of internet-users.
pissing in an ocean of piss
Doing something totally pointless.
Trying to reason with a psychopathic murderer is like pissing in an ocean of piss.