Takes place during the Cross Epoch manga special.
Luffy leapt over rocks dotting the line of a river, the first he'd seen today that contained actual water - earlier, they'd stumbled across one flowing with thick, fruity wine; the sight of Toriko submerging his entire head to get a drink had left Luffy shaking with laughter - and he shaded his eyes, searching for anything he could pick.
His gaze caught on a patch of mushrooms, delicate pink things stripped in rainbow and growing bravely from a garden of moss that reminded him of Zoro's head. Luffy snickered and jumped for them.
As his foot slapped against a boulder just beneath the jutting outcropping of rock which held the mushrooms, it gave way, unable to bear his weight. With a shout of surprise, Luffy tumbled backwards, arms flailing wildly as body connected with rushing water.
The river was much deeper than he'd though; Luffy sank several feet before he hit bottom, still struggling to swim despite his inability to do so. A strong current seized Luffy and began dragging him downstream, kicking up clouds of sand that stung his eyes. Mouth opening instinctively at the pain, Luffy choked as water flooded his throat. He hadn't been given much of a chance to suck in air before falling, and he was rapidly running out of breath.
Luffy was dimly aware of a second splash as something else crashed into the water. Then he was being grabbed again, not by the river but a huge set of hands that pulled him up and out of the sand, brought his head above water and cradled him as he coughed liquid from his lungs.
Luffy's head dropped against a broad shoulder as Toriko carried him to the edge of the river, setting the pirate carefully on the ground and kneeling beside him. One hand never left Luffy's back, rubbing in soothing circles as Luffy slowly got his breath.
"All right?" Toriko asked when his chest stopped heaving.
Luffy nodded, smiling gratefully at his newest friend. "Thank you, Toriko."
"Not a problem," he said, grinning in return. "Good thing you mentioned not being able to swim."
Humming in agreement, Luffy wrapped his arms around Toriko's waist, surrounded by warmth and happy to be alive. Fingers slipped under his sash to settle gently on his left hip. With his nose nestled in the dip of Toriko's collarbone and the curve of a smile pressed to his cheek, Luffy decided running off by himself had been his best idea yet.
A/N - I just needed this pairing in my life. Crack OTP that isn't really crack, considering they've met in canon.
Might write some Toriko/Luffy/Komatsu at a later date.
Oh, and those mushrooms Luffy went after are Gayshrooms. When eaten, they awaken you to the beauty of the same sex. Luffy never ended up picking them because he and Toriko obviously don't need such a thing. It wouldn't work on 'em anyway.