I use a 3lb dumbell. I subscribe to a rule that I was told a few years ago...if the current is too strong for a 3 lb weight to hold you, you have no business anchoring a canoe there. It may be overly cautious, but it keeps me out of trouble.

Instead of tying it off on the canoe or kayak, I clamp the line to the gunwale with a cheap spring clamp. This way I can turn it loose with a quick squeeze or even a backhand slap in an emergency.