Deano taught me how to cross white water (this is an easy one by the way - so long as you're committed). To cross fast water put yourself upriver as far as you can, aim for the far bank at an angle downstream and let the current take you. Use the current and don't fight it. This is no "one foot on the ground at all times" crossing - that's impossible - this is a run. Be prepared for your shins to take a clatter as you collide with rocks, surf over the rocks and use your free arm to sweep yourself across if you lose your balance. Whatever you do, if you lose it go feet first and tuck your knees up. You don't want to go down the rapid head first.
If you're not committed you won't make it.
And sometimes you don't get across even when you are.