Hazumu, while it's a good idea to has a lot of drawbacks as indicated by other posters. Another draw back not touched upon yet is you might have a holdup at gate. I say this because that amount of water might be considered a swimming pool if it ever breaks away from the bed. Swimming pools are on the do not bring
list. More so if you glance at the survival guide
you'll notice a trend of stuff that creates MOOP isn't allowed inside. A bed's worth of water might be considered MOOP.
Now I know you are thinking, "but don't a bunch of people bring more then a beds worth of water into the city all the time." Yes, yes they do. The issue is with the container for the water. Hard plastic or metal drums aren't going to break from the volume of water in them. But more importantly they can take passing by damage from people. Imagine if you will, you bring some hot young thing into your tent to hand them a refreshment. Upon entering your tent their platform shit-kickers with spikes at the toe glances your bed, puncturing it. Or you stumbled home at four in the morning, and plopped down forgetting the unclosed safety-pen holding your tail on your rear end from when you played pin the tail at that one camp around eleven.
Of course everything above is speculative, but worth considering. Along with everyone else suggestions.
If you are looking for that waterbed feeling, consider a double tall air mattress (the single chamber kind). Typically their rate of deflation gives the same feeling as a water bed since you sink down to about half-way by the end of the week. Also you might not need to re-inflate it.