As the name implies OZC’s Flex XLC Watercooled memory is quite flexible.  The heatsinks on them are able to function as waterblocks to theoretically give you more room to overclock, but are also quite happy to cool passively without any water at all.  As HotHardware discovered, the cooling does dramatically change the temperature, but not the maximum speed.  It is very likely that the cooler temps achieved by watercooling would significantly increase the lifetime of the DIMMs, so it is not a meaningless feature for overclockers.
‘With the release of their Flex XLC modules, OCZ is offering a product which is substantially more flexible than most other enthusiast class memory modules on the market. They can be air-cooled or water-cooled. They’ll run at lower clock speeds with tight timings, or at high clock speeds with more relaxed timings. The Flex XLC modules are unique, efficient, and from what we’ve seen thus far, the pricing is in line with other high-end modules.’

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