The nearest place for a guided whitewater trip is the Sevier River. These trips start just north of Marysvale UT on HWY 89. This is a great class 2 and 3 run, it is action packed and takes about 2 hours. This section of river is reservoir released and is typically in season through the May – August. Here is a link to the water release gauge. Ideal flows are 300-800cfs. Always Rafting guides trips anytime the river is runnable.
It is possible to raft/kayak on the Virgin River but this must be done as a private party. There are many quality sections but all should be treated with respect, it is a steep and fast moving river. Typically spring is only time water levels are high enough to run.