Water cooling is really the same as using any old heatsink. It just adds an additional step when transferring heat from the source (your CPU/GPU/whatever) to the dissipative surface (your radiator). The water is useful due to its high heat capacity.
But.
When it comes to your radiator, you're moving heat from metal to air, and air has a poor heat capacity compared to water. You really have to push a large volume of air past the fins of your rad to remove any considerable amount of heat. Given a constant heat flux (that is, constant rate of heat moving through the cooling system and being removed from the radiator surface) and a high velocity for the air moving through your rad, it's quite probable that the temperature of the air leaving the rad won't be that much higher than ambient. If the air leaving your rad is really hot, then it's a good sign that your heating system is being pushed to or beyond its limits.
The problem with using the rad fans as intakes is not the temperature of the air entering the system, but the fact that you're moving heat into the system. If you have a bunch of other components, like screaming hot VRMs, trying to move their heat into the air filling the interior of the case, then pushing a bunch of extra heat into that system makes it fundamentally more difficult for those heatsinks to do their jobs. You could be blowing air 5C or 15C above ambient into your case, but that doesn't really matter if the +5C air is high flow and the +15C air is low flow. If the quantities of heat are the same, both scenarios ultimately present you with the same problem unless you're removing heat from the case at least as quickly as you are introducing it via the rad.
If your case is a windtunnel moving hundreds of actual cfms (not just estimated cfms on paper), then the rad blowing into the case won't hurt you much. You might even be able to pull it off with no exhaust fan if the rad fans are sufficiently aggressive. Still, there are plenty of good reasons not to do this. Deliberately blowing heat into your system just makes things harder. It is my understanding that most externally-mounted rads are meant to operate independently of the case airflow system, meaning you wouldn't use your rad as an intake or exhaust element. Also, lots of people with water cooling want to avoid the kind of noise that you'd be encouraging by running more-powerful radiator fans to accommodate any airflow setup featuring the rad as an intake element. Sure, some nutjob like me would put 40-50 dBa fans on a rad no matter what the circumstances or the number of fans, but that doesn't mean you have to do it.