I'm sure you could, but you may have to invest lot of time and/or money into marketing yourself as an enthusiast gaming pc designer. Whether this is in the form of a business website, or a ad on craigslist(I'd advise against craigslist if you want to portray yourself as a professional builder)
It's just like Any business. you need to factor in the cost of a pc, the time it took to build it, the skill level required to do the custom water loop, and the cost put shipping, and from that, give yourself a profit margin you can work with.
There is definitely a market for this, but i don't imagine it will be very lucrative, as a lot of people are anti water because they think it's instant death if a leak occurs. even with a warranty, people dont want to deal with the hassle of packing it back up and sending it to you to fix.
now, another potential business would be building fpv racing or camera drones. Similar skills are necessary, and there is a Huge market for these things, because they break easily and get outdated with new tech almost weekly.