First of all - the weather looks fine so far (for northern germany
)
There has been very few rain in the last months, so the ground will be able to soak up a lot if it happens to rain.
Good Footwear (as you mentioned) is mandatory. Even if its a very dry year, there will be patches of mud. You might not need rubber boots, but you will almost certainly need solid, heavy footwear that can handle a little moisture.
Bring some kind of poncho thats foldable and fits in your pocket (better safe then sorry, wacken is between the north- and baltic sea, the weather can change drasticly within minutes)
There is no distrubution of hot water, only cold. BUT chances are nearly 100% that the people camping around you will have a camping stove. Ask nicely if they can heat up a couple 100 ml more. (In the morning you often even hear someone shout "someone need hot water?" because they heated up a bit too much.)