it may be worth noting that visualizing the ecoregions as a map background or basemap (this request) is conceptually different than adding the ecoregions as a set of places in iNaturalist (which would allow people to search for observations by ecoregion).
if you just want to visualize the ecoregions as a background, then i think the best request would be to have iNat add a way for users to specify their own basemap tiles (as described here: https://forum.inaturalist.org/t/use-openstreetmap-maps/2588/74).
if you’re wanting ecoregions to be created as a set of places in iNat, then that’s something different than what’s contemplated in this request, and you should probably make a separate request. but as bobmcd notes above, marine places have been been requested to be added, amd there’s been no action on that as far as i can tell: