I know from checking that there is a single species that can be found in freshwater irl, but considering in-game we catch it by diving in the ocean, shouldn't it be mixed in with the other diving critters and the ocean fish?
Oh, THAT’s where it is?? I was looking all over the ocean part of the aquarium, and I just couldn’t find it! Including it with the freshwater fish is…a choice.
i'm starting to hop on this theory since horseshoe crabs are not true crustaceans or even crabs, but rather they're more closely related to arachnids like spiders or scorpions
i suppose putting the crab in the freshwater exhibit would make sense if you caught it closer to the estuary/river mouth because like you stated, there are a couple species of horseshoe crabs that can be found in those brackish like areas, but even then those still contain more saltwater, just less than the ocean. little plothole i suppose!