First of all you need to make your mind open with what resolution you want to go in the next few years. A custom water cooling loop is top of the line consumer grade cooling and gives you the best looks and results, but is quite expensive in the first place.
If you are swapping out GPUs regularly (on a 6-9 month base) I would skip water cooling if you aren't an enthusiast.
Next thing to consider is that all over 1080p, no matter if 1080p surround, 1440p (surround) or even 4K you will be better off selling your GTX780s and get the 980s in a 2way SLI config. Now watercooling these GPUs with a custom loop will give you a long while a good performance. The 4GB VRAM are a huge plus and not to forget that Maxwell's L2 Cache is 33% bigger then Kepler's. I can see my GTX980's stretching their legs more in higher resolutions. A GTX980 2way SLI is a waste of money for 1080p, if you plan to play in 1080p nuy yourself a GTX970... it will be plenty of performance.
Waiting on a Titan II or GTX980Ti... I don't think we will see this cards from nVidia before AMD has made their move in terms of their next highend chip on a GPU.