Apparently at some point of the creating process there was an idea of Po rescuing (or at least trying to) Lord Shen after his ship blown up.
Not gonna lie, I'm glad they abandoned this concept. I uderstand they wanted to show Po as forgiving and not vengeful. But even if we assume Po truly did forgive, it doesn't change the fact Shen just tried conquering all of China using his deadly weapon.
Would you risk your life to save someone who not only killed your parents and made you (as Po thought at that time) the last panda in the world, tried killing you four times, tried killing your friends, killed one of the legendary masters, almost killed innocent citizens of Gongmen (when he blown up the bridge) and tried conquering whole country? Uhh, I don't think so.
Especially since there was like no chance of Shen changing, he was just evil (which we eventually witnessed when he tried killing Po for the fifth time despite the fact all his plan failed) and the only thing Po could do after rescuing him would be throwing him into jail. But again, would you risk your life with drowning (or with this someone hurting you) only to imprison someone who (let's be honest) deserves death?
Artworks by Bill Kaufmann and Sean Eckols