Author's Note: Less of a full note and more of a quick reminder that the clues and (at least partial) solutions are noted for those who wish to test their own wits against those of the characters. As always, I make no claims to the Potterverse, I just play in JK's sandbox.
Blaise Zabini did not believe in coincidences. He also did not believe in luck, serendipity, or any other such nonsense. What he did believe in was knowing as much as possible about everything, so that he would not enter situations unprepared. If he understood nothing else from his mother's lifestyle, that lesson would serve him well enough to get by for the rest of his life. So when he was paired with Mandy Brocklehurst, he checked his file of information on the girl, noted no obvious incompatibilities between them, and pronounced himself satisfied with the assignment.
When the first clue was revealed, Blaise recognized the cipher pattern immediately, and only his insistence on writing out the full answer in his meticulous handwriting kept them from being the first team out of the Great Hall. With Mandy rather content to follow his lead, Blaise took a less-straightforward path towards the docks, hoping that throwing off everyone's sense of where they were would make up for liberally using secret passages to arrive at their destination. The path was not in any way less time consuming than the main path through the school, but it offered a clearer look across the grounds to see how the rest of the field was moving.
Arriving at the docks, Blaise opened the clue, and noted as much as possible about how it was worded and emphasized. Sparing a glance at the nearby greenhouse, he issued Mandy clear and easy to understand instructions. "There will be Gillyweed inside there. Get three portions: one for you, one for myself, and one for the Merchief. If anyone enters and questions you, say nothing. Go." As Mandy ran for the greenhouse, Blaise noted that 12 boats still remained, and that Padma and Sally-Anne were still visible ahead of them, so they were firmly third at the moment, with a very good chance of making a move into second. A glance back up towards the castle, and Blaise allowed himself a moment to relax, seeing that the remaining teams would likely not make the docks before he and Mandy were away. He helped a returning Mandy into a boat, and made no sign that he recognized anyone else reaching the docks, save for a casual wave in the direction of Daphne and Tracy as they set sail a minute later.
Blaise observed Padma and Sally-Anne's boat slowing to a stop above the Merfolk village, and the girls preparing to dive. "Begin eating your Gillyweed now, and as soon as you feel the gills, head directly down under the boats. If we time it right, we should be able to swim past them. Once we encounter the Merchief, I will handle the meeting. Make no sudden movements, and everything will go smoothly."
Hearing no protests, Blaise consumed his own portion, and as he dove into the Lake, he could sense Mandy following him. All according to plan. As was their pass of a somewhat surprised Padma and Sally-Anne. That particular coup allowed them to reach the Merchief just as Granger and Weasley swam away from his seat in the center of the village. "Your friend the Headmaster sends his greetings with us, as well as this token of appreciation," said Blaise in flawless Mermish as he presented the third portion of Gillyweed to the Merchief. The Merchief looked rather impressed, and after handing Blaise a box that contained the next clue, he also handed over a shark tooth necklace as a sign of friendship between the village and the Zabini family. Blaise bowed his thanks, and he and Mandy returned to their boat.
"You did well," said Blaise as he cast a quick drying charm on both of them. "Open the box, and if it begins screeching, cast an Aguamenti into it." As Blaise had suspected, the next clue was sung in Mermish.
Clue #3: "From visiting our watery lair, you now return to breathe the air, your next task rather far from here, but once you're there it will be clear, for down the path the site you seek, that makes quite many very meek, a simple spell you'll need to cast, if you desire to not be last."
Blaise wrote the clue onto some spare parchment so that Mandy would not have to keep the Aguamenti going for too long. When the boat touched the far shore of the lake, Blaise helped Mandy out of the boat. "Do you need any sort of sustenance to boost your magic?" he asked, offering a granola bar from inside of the pack he had brought along. Grateful for the snack (as she was starting to feel a bit peckish), Mandy took the package and began munching as Blaise led her towards Hogsmeade. Eventually, Mandy did recognize where they were headed, and swallowed the last bit of granola before asking her question. "We're clearly headed to the village, but is there a specific place we need to go? I'm afraid that song twisted my brain into knots."
Glancing around to make sure no other teams were nearby, Blaise chuckled. "As for the where, there is only one place I know of around Hogwarts that students can easily get to that meets the description of a site that is sought, despite inducing fear in those who see it. As to what we do when we get there, that I do not yet know, though I expect that having your wand at the ready might be wise." Mandy pulled her wand from her sleeve, but realized that Blaise had not fully answered her question. "So, what place did you mean?"
Blaise pointed down the path that he had been leading them on. "The best known haunted house in Britain; the Shrieking Shack."
Meanwhile, back in the Merfolk Village, the team of Seamus Finnigan and Draco Malfoy had just arrived to the Merchief's throne. The Merchief stared at the pair for a moment, and when he made no move to do anything, Draco thrust out his hand in expectation of receiving the next clue. The Merchief stared at Draco's hand for a moment, before turning towards Seamus. He glanced at his teammate, who indicated his own outstretched hand, which Seamus took to mean that he too should put his hand out for the clue. The Merchief reached behind his throne, and grabbed a rock with some carvings on one side. He laid it carefully on the hands of both boys, who suddenly felt a sensation of a hook behind their navels. As they disappeared, the Merchief shook his head in disappointment.
The Portkey deposited Seamus and Draco in the middle of the Great Hall. "It seems as though neither of you paid enough attention in Care of Magical Creatures this year," announced Professor Dumbledore, loud enough for the entire Hall to hear. "For if you had, you would know that meeting with the leader of any sentient magical species and not bringing along some sort of gift is incredibly rude. I do not believe I need to explain to you that you are now out of the Hunt."
The boys shot each other looks of great disgust before returning to their house tables. All across the Hall, small sums of money began changing hands as one of the dark horse contenders fell from the Hunt. It would not be the last time money changed hands on the day, nor would it be the last time a supposed frontrunner would be eliminated early. For not 10 minutes after Seamus and Draco were Portkeyed in did the team of Anthony and Ernie appear in the Great Hall. Once their elimination had been explained, the Ravenclaw jumped his Hufflepuff teammate and began smacking him upside the head, berating him for his arrogance costing them the competition. Quite a few bags of money were exchanged during and after the fight, including one between Seamus and Draco. When more than a few people stared at him collecting his winnings, Seamus was heard to say, "If you'd heard how the two o' them were carrying on, you'd bet on a fight happening before the day was over too. Easy money." His statement was met with shrugs as people returned to watching the Hunt play out.