dissolve internal borders of polygons using st_union and group_by

Thank you very much to all :)

Barry+Roger contributions sorted the problem for the moment, although in
my mind there are still things to be understood.
Mike, yep, thank you for alerting me of rnaturalearth issues- I was not
aware of it. In this case, I am really interested in the process to
apply in other polygons, not in the data *per se*, but will take this
into account in the future.

Once I have the doc produced (distribution of world
estuaries/bays/transition waters), I will post on the list :)

Best wishes,
M.