NatilaM Ljubav! Ljubav! Ljubav!
If you decided for such an adventure, then definitely you should go in April. Then the water is highest and the most fun. It is a bit cooler but you will forget about it when adrenalin hits you. Here’s a recommendation if you are going rafting in Europe: http://www.tara-grab.com/en/00.html
The only thing I was advised that the mandatory wearing mosquito repellent stick and long socks because the boots that you will get to know the smell ;)