There’s an interesting discussion going on over on the OSM US Slack Tagging channel about how to map underwater scuba diving trails. There does not appear to be a well documented standard for mapping these as it would be difficult to follow and survey (no GPS underwater and requires advanced skill to access).
Currently, these trails are being mapped as
highway=path. See an example here, in the United States. The paths may have ropes or some other linear indicator to allow the scuba diver to follow a pre-selected path around an interesting site.
What do you think?