I would do the Epsom salts compresses with triple antibiotic ointment afterward. If you are pressed for time, or the poor dear is just too fussy, go with a slathering of ointment two to three times a day for the first four days or so. If it is bumblefoot, I would expect the limp to go away...
There are normal bumps on the bottoms of duck feet, just callouses, and that can make it harder to be sure if there is a bumble starting up.
But limping can be early bumblefoot or a pulled muscle, or a few other things.
A nice swim in lukewarm water will help a pull or a sprain. Epsom salts...