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Ah, OK - his units were in such a mess that I just assumed he'd got a /60 in the wrong place.
Look back at the assumptions again - you're assuming that even at 700 l/h water flow through the water block you're going to get a 5 deg C temperature difference across the block! That's totally unphysical - 0.2 deg C is more like it. To heat that much water, you're going to need a hell of a lot of heat - 4kW isn't unrealistic.

Note: Numbers do add up right now I take the time to go over them properly - 0.19 kg/s x 4 kJ/kg x 5k.
hf for water is about 4 kJ/kg at 300k according to my steam tables anyway, so that one got looked up right. Basically, your mass flows and temperature differences are WAY out of reality.