Water cooling has no benefits over aircooling, other than aesthetics. Some people argue that the water is a better conductor for the heat and can move more heat away for the CPU but I believe the difference is minute.
For the motherboard and CPU, well, wouldn’t you just want the most up to date build? You would most likely be looking at an i7-8700K. The motherboard is generally up to you depending on what you want from it: more SATA ports, RAM Sockets, SLI/Crossfire support. If you need to find a motherboard, use this
this site and change sort options under the motherboard section.