Lets see... shipwrecks, potentially sunken ruins, patches of sea grass where you can find Pokemon, coral reefs, random treasure strewn about the sea floor.... You sure there isn't much point? :PI do not think Mantine will dive. There is not much point in diving in alola. I think it is a replacement of Lapras in surf.
There wasn't much point to it being in Hoenn either besides as a means to access two plot critical areas and find some Pokemon or in Unova besides finding stuff you could sell but it was still nice to have.