We last year shot directly over water with lead.

The information that Demonwolf444 sent notes that bismuth is suitable for damascus guns, but can be $3 per shell. At first blush, that is awfully expensive, but we may shoot only one duck drive per day, so that might be 10 to 40 shells, depending on one's luck. Since the cost per shoot per day might be $1000 - $1500 (25 birds X $40-50 per bird), adding the extra cost per bismuth shell might not be that outrageous.