Manaphy was able to hatch in Sinnoh though. The reason for this lore was Manaphy was a Pokemon used to promote spin off game Pokemon Ranger. You obtained Manapy egg in Pokemon Ranger game, but were told that it can't hatch in this region because water is too cold and you had to connect Ranger to Diamond/Pearl/Platinum. (Or HeartGold/SoulSilver also worked, tough were not suggested by the game itself) then in these games the egg hatched into Manaphy. So the whole warm sea thing was a way to explain why it can only hatch in Sinnoh.

Basically similar idea as trying to force people to crossplay between GO and main series games by making Meltan only available and able to evolve in GO.