Yeah, after all, it's not too hard to challenge a party
with CR 0, 0 XP, 0 damage sea horses!
Ok, I'm going to completely derail my own thread to answer your challenge.
It seems like "how you interpret things" is a huge factor. For example:
Answer 1: Just pick up the eggs. If the party has at least 4 adventurers in it, start each of them next to an egg.
- Round 1, everyone use their action to pick up an egg and hide it on their person.
- Round 2, everyone move next to another egg and pick it up. Seahorse wave 1 arrives, moves to 7 eggs.
- Round 3, seahorse wave 1 steals 7 eggs and escapes; everyone move next to another egg and pick it up. Minimum 12 eggs saved; win condition.
If the party has 3 adventurers, you'll save 9 and lose 7 on round 3, and seahorse wave 2 shows up. On round 4 if you won initiative against the seahorses, you can take 3 more and win. If you lost initiative, you will need to kill at least 5 of the wave 2 seahorses on round 3 with a maximum of one person (AoE spell perhaps?). That will save only 8 eggs on round 3, but on round 4 only 2 more will get lost, and you can use your remaining actions to pick up 3 more eggs - 11 saved, win condition. (If the party has 2 people, you lose.)
But what if you can't hold 3 eggs on your person? If you can just kill the first two waves of seahorses as fast as they show up, they cannot steal more than 7 - win condition. If it takes your party longer than 2 rounds to kill them, you'll need to make sure at least one person can get bitten by the shark each round while you continue to kill seahorses (preferably anyone who can soak 22 damage in one hit). Once they're dead: win condition. Nothing said the shark won't just leave once the condition is met.
Killing the seahorses is either really easy or impossible. If they all show up in a clump (DM's choice), they can all be killed in one AoE each round. If they are spread out to all four sides, you'll need potentially more than one attack by each player (depending on number of players) that also cannot miss an AC of 11 to kill them all - probably impossible, unless everyone happens to have AoE / multiattacks (like a fighter / monk / caster group) and lucky rolls.
But what about spell casting underwater? Well, what about it? I couldn't find anything that says you can't. In fact, given that the PHB calls out that "the words" aren't important, just saying them - I would think there's no reason to think it wouldn't work. So again: you either win easily with AoE or you lose.
Ok, ok, but let's say you can't hold more than one egg, and you can't cast anything with verbal components underwater, and you want to be practical and not attract the shark because it's not just the win condition but also escaping that you hope to do. Then you can hope you have a caster with minor illusion, which is about the only spell you'd have access to at this point in the game without a verbal component that can do literally anything. Create a huge sound that scares off seahorses. Win condition. Or possibly create something not bigger than 5' that also scares off seahorses. But that also doesn't work? Well we get back to party size again; it MIGHT be possible for you to stack at least 11 minus party number of eggs into a 5' square and cast minor illusion so that the square looks like something that's not eggs. Seahorses have a -5 to their Int check to disbelieve you, so considering you likely have a DC 13 spell save, that's a 15% chance to disbelieve. So everyone grab an egg, then stack the rest (so that a total of 11 are saved) into that square and cast minor illusion on it. You may also need to kill the first wave of seahorses, since a seahorse only needs 2 rounds to successfully steal an egg, so that would give you 5 rounds to hide them instead of 4. For a large party this probably isn't hard, but for a small party it might be impossible.
Ok, ok, ok, so your DM decides that none of that works? Start everyone near the statue and hope you can roll well. You have 4 rounds until you lose, or 5 if you take out at least 5 of the first wave of seahorses (but that will take more actions, so it's probably not worth it unless you know you can kill them all in 1 action). Everyone make a check, starting with the (hopefully free action) one to determine how to make the other checks. You could win in 1 round with only 3 people, or you could lose, depending on your rolls.
Overall, the shark isn't really a big deal: it can only down a single character per round starting in round 3 at the earliest, and the whole thing is over in a max of 5 rounds (potentially one or two more if you took a while to kill seahorses). Unless you rule that the shark sticks around, in which case that's not a practical option regardless. Fighting the seahorses is either simple or nearly impossible, depending on your spellcasting interpretation, especially since melee attacks have disadvantage underwater - and the "breathing underwater" thing is again not a big deal, since the whole thing is over in 30 seconds. The trick is that the seahorses only need 2 turns to steal an egg. Technically 3, but you'll never catch them - not when you can only move 15' per round and they can move 20'. The whole "skill checks" thing is really binary as well; you either make 4 rolls in time or you don't. It seems like everything to do with the fishing boat or the shark skeleton is a trap; you just can't possibly do something useful with those fast enough to stop the seahorses.
So: lots of ideas, but mostly depending on how the DM rules - or you can just make a bunch of checks quickly.