That is because on the Nforce 2 the chipset is not built into the CPU. The chipset on Nforce3 boards is in the CPU, the chip on the motherboard is actually the south bridge. Now thats not the exact explination, but for all intensive purposes the chip you see on the mobo on Nforce3 boards is like the southbridge on socket A boards.
Now, it would not be benificial to watercool it as it would only restrict flow. A passive option like a Zalman is more then enough. If you really must have some good cooling, get the Swiftech chipset cooler.