When shooting steel you can sorta consider your choke to be one size tighter than it is; ie, a modified is like a full. Also BB is the largest you should shoot through a full, generally speaking.
Carlson's/Cabela's choke tubes....they're only 30 bucks. Now, I only have 1 of them, a full I use in an 870 for geese, but it shoots great patterns (85+%). I still wouldn't overlook your factory modified for waterfowl. You hafta put it on a patternboard to know how it's really shooting. My longest kill shots have been with a modified and BBBs.
You also hafta pick the right ammunition. The cheap stuff like winchester xpert steel is about all I use on close range ducks. Heck, I'd use 2 3/4" 3s on close range geese. But anyways, the cheap stuff has a lot of misshapen pear-shaped pellets that won't carry energy or pattern like more premium pellets will.
Bottom line is take the time to pattern your shotgun with different ammunition and chokes so you can figure out what'll give you success.