I don't know about right around Whistler but up in the mountains behind Pemberton there is some pretty nice trout lakes. Particularly a few around the small mining town of Brailourne(sp?). Guides take folks belly boating up there. July is pretty late but there's always lots of eager tiddlers.
Mid summer most interest turns to salmon fishing in the ocean, unless someone is willing to let you in on there secret summer run stream.