I guess the creature would have to have a very big mouth for its size, and a very wide throat for its size, and a very big stomach for its size, if it is less than hundreds of feet long. Maybe if it is hundreds of feet long too.
So the log cabin floated down a river in a storm into a huge lake or the ocean, and the man sort of steered the log cabin toward an island somehow, but the island was actually a giant water monster that swallowed the man, the cabin, and a lot of water and air in one huge gulp.
The air in the stomach might make the creature unable to dive, but maybe the creature likes to float at the surface for days or weeks at a time. And maybe the stomach is on the top of the creature, with water at the bottom of the stomach and air on top. And maybe the stomach and large parts of the creature are transparent and there is light in the stomach for the man to see with, or he has flashlights or something to see.
And maybe the cabin was not swallowed by a mouth, but by some other orifice that leads into a different system than the alimentary canal. Maybe the body system was some sort of siphon for propulsion like squids use, & the cabin is stuck in a large sac storing water and air for later use in propulsion.
Or maybe the cabin is stuck in some vast space full of water and air inside the creature that the creature uses to generate very loud sounds to communicate with others of its kind across vast oceanic distances.