hey i know I'm so late seeing this thread, but eventually cement ponds crack like the erosion and constant pressure of water wears it away over time so after maybe five years of having a cement pond it can start leaking and you won't know where your water is going
Cement ponds usually have a layer of pond liner sandwiched between two layers of cement. Koi filtration will also work; you will just be cleaning filters more regularly and the same with water changes. Best method is a gravity fed bottom drain. I'm currently having to clean my filters every fortnight. Plants are good but in order to utilise them you will need a bog type filter that is covered with mesh so your ducks can't eat everything!
This was a few months ago - it's completely clear water at the moment and I'm not even gravity fed yet. This is a 2000 litre pond for 7 ducks. Edit - vid of plant destruction; a day later I had a planter with stones in the pond- they'd eaten the lot!
From what ive read a drain is the best way to go, rather than buying an expensive filter only to have it clog and die (like mine did). With a drain you can empty it easily and use a hose to spray it clean and refill it.