The amperage on those pumps is generally run less then 1amp for those 115/120. So the cost in power would be the same as a 12v pump running 3 or more amps plus the inheritly inefficient flaw of converting 115/120v to 12v DC with power supplies. There will be exceptions to this, but the price in power is basically the same. Unforunately yes, those Y splitters are very dangerous. It might work well on test after you made sure there are no leaks. But then when you goto install one might unintentialy overtighten the clamp and put micro cracks in the splitters. You might not even see the leak right away. On the other note, in order to avoid that crack, one might not tighten the hose enough and get a leak that way which will not be seen till the power button is hit. Gawd forbid if it leaks onto the computers guts.
All this is bickering is pointless tho. Watercooling no matter how careful of a job is done is vastly more dangerous for a computer then air cooling. That said, the rewards are worth it, but by spending that mere $5-$20 more, one is left with far less possibilities for things going wrong. And $5-$20 more for anyone doing water cooling will not come remotely close to making or breaking their wallet, or else they would go with air cooling. Why not just take some risk out? You might be an excellent driver, heck, maybe even the best in the entire world bar none. Does not mean you don't get auto insurance?