You should have seen the one that got away…

It might not be an actual sea monster, but this big critter that washed up near the mouth of the Murray River seems like a good enough reason to avoid the ocean for a while.

The gigantic body of a sunfish was found washed ashore by a fisherman on the Coorong over the weekend.

The man's partner, Linette Grzelak, posted the images to Facebook, admitting that she initially thought the carcass was a fake - a reasonable enough reaction to terrors from the deep.

As well as its size, the sunfish is known for its flat body and unusually-shaped fins.

Horrifyingly, a fish expert from South Australian Museum, Ralph Foster, said the Coorong specimen was actually a smaller example of the species at an estimated 1.8m long.

"They can get nearly twice as big as that," he told the ABC.

And while the fish are docile and do not harm swimmers, they have been known to damage boats due to their sheer size.