I wondered if there was any interest in adding marine features to OSM, or if there was a more appropriate equivalent ("openmarinechart" ??).

I have done some charting of small areas (too shallow/small to be of interest to commercial vessels) but there had been no way to share the data, or even to get it into my GPS as a map (MapDekode to Garmin, ugh!). Generally it's readings from an echosounder, hopefully corrected to Low Water by accounting for tides and transducer depth. I used manual entry, but I presume there are automatic solutions. In OSM, I guess one could add a spot depth as a point with appropriate attributes (units, datum, value)

I also wondered about generating terrain maps from GPS elevation data; there must be quite a lot of user data available now.