How hardcoe rates work is that it gives you a value, in seconds, that gains you 1 point of that rate.
So, for instance, you get dehydrated at a rate of 1 for every 10 seconds, by default. The higher the value, the longer it takes for you to raise that stat. That's why sleep deprivation takes forever and a bit to raise, because the value is so darn high, and the amount you reduce it by by sleeping is high.
Yeah, I just checked the Fallout wiki. Dehydration is 12 per hour, but that's a game hour. Like you said, it's 1 per real-time 10 seconds, which is 360 per real hour. Yikes.
Edit: But I also read about the Vault 13 Canteen, which seems like it would help...
Edit 2: Actually, if the canteen restores 25 hydration every five minutes, that almost completely offsets the dehydration meter.