Honestly, a duck wouldn't even have an attack, I don't think... their claws are pretty much harmless, and their bill isn't exactly any kind of threat, either. So, like a toad familiar, they just wouldn't have an attack. I'd say a ground speed 15 or 20 (a hawk has a 10 ft. speed, and they're mostly fliers, while a duck is much more ground-based and can waddle pretty well when they get up to speed), with a fly speed of 30 ft. (average), and a swim speed of 10 ft. Tiny sized, so 0 ft. reach/2 1/2 ft. space. For stats, maybe match a viper (which are semi-aquatic and have a swim speed), since those aren't a huge deal, or whatever familiar-type animal you prefer. Feats, maybe Alertness and Improved Initiative (if you ever watch a duck hunt, they seem pretty alert and quick to react), and maybe also the Hold Breath (Ex) ability. As for that they grant their master as a familiar, maybe +2 Swim?