The confection that Spock toasts at the Yosemite campfire in Star Trek V: The Final Frontier, having studied the custom and learned its name slightly wrong.
The marsh melon is what Spock calls a marshmallow in the film Star Trek V: The Final Frontier, set in 2287.
On shore leave in Yosemite National Park, Kirk, Spock and McCoy sit round a fire after a supper of McCoy's beans, which are flavoured with a secret ingredient that turns out to be bourbon. Spock announces that he has consulted the ship's library on camping customs and produces a dispenser, from which he takes a marsh melon and impales it on a stick for toasting. McCoy is delighted and does not correct him.
The three then attempt to sing Row, Row, Row Your Boat as a round, which baffles Spock. The campfire scenes open and close the film and are generally regarded as its most successful moments.