Answer: And some went fishing.
Explanation: This line doesn't make any sense. You should change it too, "and some of them went fishing", or just get rid of some entirely, so it would say "we saw the ducks fly, swim, and went fishing". Oh, and another thing, there is no capital in the beginning, so you should change it too "We saw the ducks fly, swim, and went fishing". There are more ways you can change the sentence. Like "We saw some of the ducks fly, swim, and trying to catch fish", or "We saw the ducks fly, swim, and some where even fishing. There are many more ways you can change it, ask me for more help if you need it.