Just doing some playing on TI while preparing a proposal & found three, apparently related, but undocumented & very strange (to me at least!) tags, used several thousand times:
marine=N: overpass turbo
marine=Y: overpass turbo
MARINE=N: overpass turbo
Some examples:
All of them seem to be tagged in similar ways.
Anybody have any idea of what they are?
pdnzjr didn’t reply (yet), but other users using the same set of tags did.
It turned out that they are “leftover Garmin tags” that safely can be deleted!
Thanks!
Saw your same comment on Discord. As I asked there, could this be an Auto Edit, done by somebody who knows how, which let’s me out!
I’ll happily go through & clear the info off the 12 or so marine=y, but a few 000 of each of the others is a bit beyond a manual fix! 
If we look at version #1 in the history: we see that it was added 9 years ago.
Obviously all the other later editors of that same way, didn’t see it or didn’t care or didn’t know what to do!
I notice 14 tags that could be removed as they have no function whatsoever in OSM.
I’ll see if I can come up with some structured removal, but if you want to do some clean-up in the mean time: go ahead!
With my DWG repair account I have fixed a number of the changesets that had those tags next to a number of regular tags.
I removed the zipidx, routeparam, roadid, regioname, countryname, marine, cityname, mp_type tags, as well as the nodX (x a number) tags.
They are not all done yet as I have a few questions about what I found and have contacted the relevant mappers.
To me as a Garmin user, they don’t look like Garmin tags - more like leftovers from an original import, as previously mentioned.
They still don’t really make sense in OSM, of course…