Yudishtira
Spiritual/Creative
Same should apply to harvest Sea Grass, as with kelp.
Then what about beacons on Reefs and Coral Reefs?
There is only one harvest action at sea. Basically, if there's sea grass or kelp on the tile, you can harvest "kelp" there...
Thing is, it occurs to me, in all these cases (except breaking ice), the improvement will generally have been built long before there are modern workboats.