Hello Martin,
maybe someone wrote you, so you do not have to read this.
I think, you do not need any reservation. There is a camp, where you can sleep in your tent or you can choose a cabins (then you need a reservation).The place is called Maly Ratmirov.
You can not use rafts, only kayaks and canoes. If you want to take a part in CPV, you can make your registration during the evening before the competition, or in the morning too. More information will be later. Look here - hamerak.boucek.cz
Of course, there is symbolic price for the day on water - like 60Kc and ussually the same price for bus, you can use to go back from Ratmirov or Jindris back to Dvorecek. You can go 1 or more times during the day by this bus with your boat of course. It is better to use the buses than your own car because of traffic jams.
Well, I am not an organisator, only participant. Hope, I gave you good info, if not, someone else can help.