Good point. To be honest I have no idea what kind of payload UAVs are capable of carrying. I know that there are many types, and that we've used them for cargo airdrops and carrying missiles, but those would of course weigh less than a lot of water at a time.

A quick google search for heavy-lift UAVs found a proposal for an aircraft that could carry payloads of up to 4500 kg, but no news on whether these were actually built. At that rate you'd still need to make at least ten passes a day to just get to normal cooling standards (I think I read somewhere that you'd need 50 tons of water a day to keep a reactor at decent temperatures). So maybe it's not that practical.

I know that they're doing their best out there, but I can't help but wish that our technology had more ways to protect us.