Steve Pocock, landlord of the Fountain Inn in Trellech, Monmouthshire, Wales, said about 4ft of water entered his pub at 11pm yesterday (December 23).
The pub has lost all of its food bookings, which will be 'financially crippling', he said.
Pocock said: "The fire brigade came out last night because the pub was underwater downstairs, everything was completely flooded.
"We open at lunchtime today and people won't have anywhere to sit, but our customers are really good. We've lost all of our food bookings. You could have sailed down to the pub."
He added: "You can't estimate the money lost because you don't know how much people would have spent. The next three or four days over Christmas will be spent scrubbing and cleaning ready for a party on Saturday."
Rock near Blackwood
Another pub in Wales, the Rock near Blackwood, was also hit by the bad weather.
Licensee Paul Taylor said: "We had sandbags at the pub. The council turned up with heavy equipment to clear the drains. I have been here seven years and I have never seen this much water.
"The road was was blocked for about three hours. The flooding started yesterday morning and finished last night.
"It was a bit of a blip. We are open tomorrow for Christmas Day."
Pubs across Scotland and southern England have also been affected by the poor weather conditions.
A tree also fell just three metres away from the Green Rock Tavern in Blackenhall, Walsall.