Hello everyone,
I have been using osm for quite a long time, but one thing I always was willing to have is the an “automatically generated/or computed” zone 300m from the shore that indicates the zone where a speedlimit is in place.
I have no ideas how this should be mapped, and if it’s even possible, but I generated what I think would make sens for me as user (with the help of an AI 
) Lac Léman - Speed Zone
Open for any suggestions or remarks.
I’ve never looked into it myself because it already existed / had been created by someone else, but perhaps the system on Lake Constance might serve as inspiration for you: Way: 416771494 | OpenStreetMap
The user Krillle is still active; perhaps he can tell you how he created the line.
See also the wiki:
Lake Thun has this mapped as Way: Uferschutzzone (448801537) | OpenStreetMap, but I don’t think this should be in OpenStreetMap as a way, as I think this can “easily” be calculated (without AI).
An example would be using 10. Vector Spatial Analysis (Buffers) — QGIS Documentation documentation with Relation: Lake Thun (1682683) | OpenStreetMap as input.
I got a chuckle out of “note=[…]Do not delete! Rendered in OpenSeaMap” there, as none of the available layers on osm.org render this and it reminded me a bit about “tagging for the renderer”.
Thanks for the interesting comments and hints.