I've only done that once (most of the creeks aren't really that big where I hike - like the overall size of the backcountry, things are significantly smaller in my piece of the east.)

The one time I did was, of course, in front of about a dozen folks in a beginner's trip I was helping with. The creek was about mid-calf when we crossed the day before, to camp in that near-ideal campsite on the other side. Naturally, it rained and the creek rose to knee-deep or slightly above. So, I waded about halfway across, to provide reassurance to the other folks as they crossed. (It wasn't flowing any faster than the day before, and wasn't treacherous, but they were beginners...) Anyhow, after they all crossed, I waded on over - and stepped squarely into a hole, went in over my head, and popped back up. Nothing in my pack got wet - which completely surprised me.

There really is no good way to guarantee a dry pack - just some ways that are less bad than others. "Dry most of the time; not fatally wet ever" is about the best we can hope for.