Tom Pickles, 24, said he snapped the picture of the creature, known as the English counterpart to Scotland's Loch Ness monster, Feb. 11 while on a kayaking exercise with his company, The Daily Telegraph reported Friday.
Pickles said the creature was the size of three cars and was visible for about 20 seconds.
"Its skin was like a seal's but its shape was completely abnormal -- it's not like any animal I've ever seen before," Pickles said.
Locals said the picture represented the eighth Bownessie sighting in five years.
David Farnell of Farnell's Photographic Laboratory in Lancaster, England, said the picture appears real but is difficult to verify due to its small file size from being taken on a phone.
Ian Winfield, a lake ecologist at the University of Lancaster, said the sightings may have been large catfish and the viewers are "misjudging" the size. He said there are no known creatures in the lake matching the reported size.
"We run echo sounding surveys every month and have never found anything," he said.