Some more context that won’t be told through dialogue:
Izuku and Inko used to live very close to the surface since most coral reefs aren’t usually deep, and in naturally warm waters
This ended up doing 2 things, 1- encouraged humans to make the reef a “ tourist destination” with the Coral reef being the center attraction. And since their waters were almost always warm, humans actively sought it even during colder weather, such as the time Katsuki and his family left for migration. This caused the location to spike in popularity
2- All of Izuku’s pod are only used to warm waters. Due to this, drastic temperature changes from fleeing and traversing into much colder waters, would cause their bodies to go into shock. Ignoring those who fell prey to other species, only the very young and healthy of his pod could survive and make it down to the Shark Mer’s territory
Izuku lost more than half his pod between those that couldn’t make the trip and those that left and attempted to return home
For those who don’t know (as I only mentioned it as a tidbit in the bonus chapter) Shark Mer are incredibly speciest and view all non combatant type Mer as lesser than them. Meaning Izuku’s pod was only accepted if they were prepared to be treated as a lesser class. It is based on a hierarchy, but their discrimination is not limited to just Regular Mer
However, this is true for most Mer groups in general, Shark Mer just being one of the worst. In general, species do not like to mingle and certain groups (such as Cephalopod Mer) see other Mer groups only as a food source. So Izuku and Katsuki’s relationship in this story is very unique even by this AUs standards