I've recorded a sailing activity using the pre-defined "Boat" profile on my Fenix 5 Plus. After it was synced with Connect on my Android, I edited the activity to turn it into "Sailing". However, instead of 16.71 NM it suddenly showed 16.71 km.
When switching the type from Garmin Connect Web, the value is converted correctly. But still, I think a "Sailing" activity should show distances in NM.
Also when editing a Sailing activity with the Garmin Connect app, I don't have the option for "NM", only "km" and "mi". It seems as if "Sailing" is not recognised as a maritime activity.