Maybe your herbalists keep failing to pick the rope reeds? DFHack can tell you how many of each plant is on the site.
It was indeed a herbalist fail, due to my extensive lack of luck. After using DFHack to pinpoint their location, my herbalist failed to successfully harvest any of them. TBF, the guy never had any real training, I don't rely on them for food.
perhaps the ocean biome thing meant that if you somehow manage to get a farm above the ocean it won't work?
The wiki page used to say that "Above ground crops farming is impossible in embark sites that
include Ocean or Mountain biomes, even if the farm is built on mud over a soil layer, in an area of the map where trees and shrubs are currently growing wild (and thus are
not technically ocean/mountain biome tiles)." This seems to indicate that farming is impossible in the
whole embark if there is a single piece of ocean involved, which is not true, at least for oceans: my rope reeds matured just fine. It remains to be tested if plants can be grown directly above the ocean, but my current megaproject includes one such farm on an "oil rig" fortress, so I'll be testing that within a couple of days. I just have to complete my magma-redirecting and ocean-lifting projects first to manufacture an obsidian pillar directly above the ocean, which I'll then proceed to cave-in to have a 3x3 pillar connecting the ocean floor and my fortress.