I tried it a while back when using a Reserator - 2.5L capacity.
It is a great idea if your load profile is generally low with short spikes - the water doesn't have time to warm up so the spikes are kept pretty cool. However if your load profile has a large amount of time at high load then, as others have pointed out, it will not really help much as the water will eventually reach the same temperature regardless of the amount of water present.Except!
Having more water means that you have a larger reservoir (or a Tardis), and the larger surface area of the reservoir can be made to help cool the liquid. The effect is generally minimal (the Reserator has abysmal performance compared to a 120mm rad and fan) but it can help out a bit. Basically your res can become a passive rad.
It is possible to make a loop entirely cooled this way however you res would have to be huge, fairly thin and flat (cylinders have a poor surface area / volume ratio) and metallic (plastic is a good heat insulator). Something like a large household rad might work well.