No you do not need water cooling. Personally with the air coolers on GPU's anymore water cooling is not really needed even with SLI. As for going for a SLI setup in the first place really depends on what resolution and the amount of monitors you are running. For a single monitor at say 1080p running a single GTX 780 really does not even need a SLI setup because there is more than enough power in the single card to do what you want even at max settings. Running a Surround setup is where a SLI setup would come into play.
Well, right now I've got an Asus VG248QE, and I'll be upgrading to the ROG Swift monitor when it comes out. So basically, I'll be gaming at 1440p with 144hz refresh, so I want the horsepower to take advantage of that. Single monitor for now, sometimes in 3D via Nvidia 3D vision. I'm new to gaming builds, so thanks for any guidance you provide.