Question

Why might it be hard to survive as a duck with web-less feet?

Answer 1

It's solely because of ineffectiveness while swimming.

Just recall what the sea divers use while swimming,it lacks the web space,unlike our toes,just because you can apply a large amount of force on water to create a good amount of thurst,so that the reaction force will be enough to move forward.

If web was not present,while pushing water behind with toes,between two toes,water would have been entering through the gap infront,reducing the effectiveness of pressure.

The same is with ducks,they need web in place of web space as we humans have,as they spend a lot of time in swimming in the water.