* Seahorses are very slow swimmers. They use their fins to propel themselves in a vertical position, making them quite inefficient.
* Sharks are generally fast swimmers, but there's a huge range of shark species. Some, like the great white, are incredibly fast, while others are more sluggish.
* Pufferfish are relatively slow swimmers. They mostly rely on their ability to inflate themselves to deter predators.
So, the answer depends on the specific species of shark we're talking about. A great white shark would easily outswim both a seahorse and a pufferfish. But, a slower shark species might be outrun by a pufferfish.
To be precise, we'd need to know the exact species of shark being compared.