Your sentence is mostly correct! Here's the original sentence:
Like many water birds, Mandarin ducks have webbed feet that work like natural flippers, helping them swim quickly and smoothly. These special feet also let them dive underwater where they can catch fish.
It's clear and well-written! If you'd like, here are some tiny improvements to make it flow even better:
- After 'underwater', it helps to add a comma for clarity, so: 'underwater, where they can catch fish.'
- Also, you could add 'which' before 'helping' to make the sentence slightly more formal: '...natural flippers, which help them swim quickly and smoothly.'
So a revised version could be:
Like many water birds, Mandarin ducks have webbed feet that work like natural flippers, which help them swim quickly and smoothly. These special feet also let them dive underwater, where they can catch fish.
Great job on your writing!