Marinecoin doesn't have a Airdrop but a free ICO right now which is pretty much the same i guess. All you have to do is help a little with the promotion and you get a ton of free coins. Also this might become a giant in the near future since it has really promising tec, extremely fast transactions and great scalability. You shold definetely check it out:
https://marineco.in/icoSo it works on a referral basis? I'm too lazy to do the research myself, sounds pretty fishy at the moment.