Haven't been there but studied traveling there a bit.The best locale for whitewater seems to be around Merida in the Andes it is the outdoor sports hub of VZ.There was a magazine article a few years ago,AW? or Kayak Session?,about some boaters doing first D'S based out of Merida,I'll see if I can find it for you.I met a Kiwi in Guatemala who was going to work for an outfitter that did whitewater near Merida,raft trips on the Orinoco,and sea kayaking in the Caribbean,so there are outfitters try a google search.Another mag had an article about running big water with some 4-5 rapids on the Caroni the river of Angel Falls or very near it,in a Prijon Yukon.Check a recent Lonely Planet for outfitters.
The whole Guyana Highlands area is one of the least explored areas on earth.I THINK IT IS FULL OF RIVERS TO EXPLORE encompassing SE VZ,southern Guyana and Suriname,and parts of No. Brasil.It is a real dangerous area,aside from remoteness and serious jungles and sabanas you have border disputes ,drug running,and political unrest.If that is not bad enough it has abundant crocs,caimans,GIANT anacondas,and bushmasters!!!