Cheshire Fire Service sent six fire engines to the blaze at Neston after it started just before 6.30pm on Saturday.
The fire covered about 1 sq km (0.4 sq miles) and was “mainly extinguished” by midnight, the fire service said.
It contacted police and a fire investigator as it suspected the fire may have been started by someone.
Eyewitness Peter Guy, 43, who was visiting nearby Parkgate on a day trip, said he had just finished having fish and chips when he noticed the fire.
He said there was no explosion, but the fire took hold rapidly, creating a “wall of flames” across the marshland.
“It started off looking like a regular bonfire and then I noticed the fire getting closer to us,” he said.
There were so many people there enjoying the beautiful sunset. I’ve never seen anything like it. It was like the sky was on fire.”
On Saturday evening, the fire service said there were two areas of fire spreading around the marshland, a site of special scientific interest.
Fire breaks were put in place to protect nearby properties.
By midnight, all but one fire engine had been withdrawn to safety as the tide came in, with the fire mainly extinguished.
By 7am on Sunday, crews confirmed the fire was out by carrying out a sweep of the area with a thermal imaging camera.
