Overview
Swamp Thunder is a fan-imagined airboat pursuit through Grassrivers. Trailer coverage confirms the swamp region, complete with alligators, hillbillies and “weirder discoveries,” but no airboat mission by this or any name has been shown.
The theory
Cal Hampton, Jason’s laid-back friend who lives at the low tide of America, gets floated as a giver for low-key rural jobs. That is character-fit guessing, not a confirmed assignment.
Why people believe it
The marsh terrain looks like nothing in past GTA maps, so fans assume Rockstar will build dedicated swamp gameplay around it. A plausible assumption, but the specific chase is invented.
Gallery
Grassrivers marshland frames from the trailers, archived as setting reference.
Trivia
“Swamp Thunder” won a community naming straw poll over “Gator Run” and “Marsh Madness,” on the grounds that it simply sounds the most like a real mission title.