Truthfully, I feel like Island Living and Mermaids should have been separated. So much gameplay was put into environment conservation that it was likely impossible for EA to put more into mermaids. If they had been given their own pack, then I could see the power system being possible. It would be nice if they did make an actual Mermaid pack, really. Island Living could still have mermaids for those who'd just like to have them around or create in CAS but don't intend to use them much for gameplay. But a Mermaid pack could finally give us a completely underwater world, have a power system, lore regarding mermaids and maybe a small world they can go to above land - it would work like how Outdoor Retreat has the deeper part of the forest where you find the hermit.
If that were done, then I could see the things listed under Island Activities being more possible.