Your sentence is almost perfect! Here's a corrected version with some small grammar tweaks for better clarity:
Corrected sentence: Like many water birds, Mandarin ducks have webbed feet that work like natural flippers. These special feet help them swim fast and dive underwater, where they can catch fish.
Explanation: I added a comma after "underwater" because the phrase "where they can catch fish" is extra information explaining what they do underwater. This comma makes the sentence clearer and easier to read.