This admin area http://www.openstreetmap.org/relation/272632 has both the coastline and a maritime border. I guess only the country should have the seaborder? Or could there be a local convention as to what to use?
> I guess only the country should have the seaborder?
This might be country specific. It depends on which administration is responsible/managing the sea area. Usually this is more at country level but it migh be different here (to be checked).
But what the wiki says about maritime boundaries:
is that :
" If the land_area is not the same as the boundary, make two relations, one with land_area=administrative and one with boundary=administrative. "
So the relation should not contain both, coasline and maritime border. This is normally done in two separated relations.