Since you know they will immediately come to repair the dam as you break it, I would set several 330s where you think they will approach from to repair the dam. Using guide sticks to reduce the area they have to swim in helps. It doesn't have to be an actual fence, just enough to guide them through were you have a 330 set. Diving sticks can help as well. They are on a mission to repair it, I would not worry with castor or lure in this situation.
Also, 330s eliminate all the fuss with having to try to drown the beaver.
It might take awhile but when you have beavers readily coming to do dam repairs, you should be able to catch them all.