The bones rattled loud against still water as the ghosts of the undead human army rose. They remembered their purpose, their valor and honor, and their racist desire to rid the world of the undersea creatures. They were affronted by their deaths, by the loss against the merpeople. Growling and hissing, the ghosts traveled through the water with ease, floating through the top of the cave and out into the lake proper. They were weak ghosts, not able to kill, so Harry was not technically committing the mermish genocide he'd been denied.
Harry waited until the noises ceased and all the ghosts had left before slipping out of the black cave. Plausible deniability was important after all. Checking his watch, he saw he still had forty minutes. He swam around a bit, doing some sightseeing as he headed deeper and deeper toward the middle of the lake.
It was eerily quiet. The ghosts had spread out to terrorize the locals, scaring away the wildlife as they booed and gurgled and chased down the mer population. And so Harry had a peaceful swim down to the edge of the mer village. He swam down a path surrounded by stone buildings illuminated by glowing crystals. The whole place was abandoned. There were objects left scattered, still floating down to the ground. Bits of food and a doll made of sticks and seaweed.
His distraction had worked nicely. Harry grabbed a spear that had been lodged deep into a stone wall, a desperate attempt to attack their surprise besiegers no doubt, before the merfolk had realized their foes were incorporeal. The spear had a large, handcrafted head of some dark metal. It was very cool, and Harry took it for no other reason than as a souvenir.
In the middle of the village, dangling in the water from ropes tied to their ankles were three figures. The shock of carrot hair waving about was instantly recognizable. Oh. That explained where Ron went.
He hefted the spear and cut Ron's ropes with care. He wondered idly why nobody had thought to have the three hostages change out of robes. Why did Ron need his shoes for this? There was a Durmstrang student to the left who looked weighed down in his heavy winter coat, and the last person, a small girl - Fleur's sister he recognized, had an elaborate outfit on. Harry took care to not cut into Ron's socks. Just as he freed him, Viktor came swimming up, half-transfigured in a rather unsettling combination of man and shark. Viktor, who was surely thinking the same thoughts Harry was about how to free his friend but with bulky teeth, accepted Harry's offer of the spear gratefully. Harry sawed while Viktor held his friend steady, and just as they were finishing up with that, Fleur swam over. She had a cut across her cheek and was breathing heavily, but gave the boys a reassuring gesture when they both gave her concerned looks.
"Race to the top?" she shouted behind her Bubble-Head Charm.
They burst through the surface around the same time. Krum first, with his Quidditch stamina and powerful fins, Harry a few seconds behind him (Ron was heavy), and Fleur right behind him. The cheers from the audience could be heard even from their side of the lake.
Harry, having forgotten that he could not breathe air until his hour was up, gasped and floundered until someone hit him with a spell to cancel its effects.
"Er, thanks," Harry said, once he was no longer fishy.
Their hostages had woken, furthering Harry's confusion about the lack of swimwear as Ron treaded water as he tried to remove his shoes. Fleur's sister was clinging to her and the Durmstrang boy said something in Russian, curses from the sound of it as he struggled to keep his head above water.
"Take the coat off," Krum said as the group began to make very slow progress back to the dock.
"Expensive," the boy growled. "It is sable!"
"Does anyone know how to conjure a boat?" Harry asked.
"Here. Give shoe, can transfigure," Krum said. Ron did not protest. Although Harry figured Viktor could have asked for Ron's wand, and the smitten boy would have handed it over.
The boat was only a little bit leathery but it fit everyone and with a slash of strong wind, courtesy of Fleur, they returned to the docks.
Everyone was bundled into towels and given drying charms and Pepper-Ups.
"Harry," Ron whispered, his skin still flushed red which was causing Madame Pomfrey to fuss over him. "Harry! Viktor Krum transfigured my shoe!"
"I was there," Harry said. "I saw Ron. I suppose you'd better transfigure it back and get a display for the thing."
"Wonderful idea!"
Harry laughed as they all huddled around the judge's platform to await their scores.
The judges were a bit confused as to what had scared the undersea population off. Only Fleur had encountered anything, a small group of grindelow that had fled amid their attack. They had convinced the merpeople to provide some suitably violent entertainment, but there was also no proof that anything had happened to them. It wasn't any of the contestants' fault, the event had simply not been as exciting as expected. Although perhaps with all the drama about safety afoot, it was best that the task had been on the milder side.