In Wizard City, in Old Town, there's a small stream next to the first crafting quest giver. You used to be able to walk through that stream.
However after the last update it has become impossible to do so, and there is where the problem is. When the stream was made off limits, the respawn spots for the cattails weren't moved. So the cattails are spawning in places that are no longer accessible.
You can still access that cattail. Simply walk around behind Eudora Tangletree and around to the backside of the building. You can harvest the cattail from the edge of the bank there.
If memory serves me well, I believe there were several different spawns for cattails in that pond. I'm not sure if there were two or three, and if I remember correctly, they were mostly in the corners of the pond. It has been a while since I was at that particular pond.
After last week's update, was the first time I had been there in a long time. The cattail I harvested at that time was directly behind Eudora and close to the front side of the pond. It is quite likely that you are correct if the other spawn points, being farther away from the bank cannot be harvested. I will have to check it out next time I am online. My apologies for forgetting there was more than one spawn point there.
You are indeed correct, there is a cattail that is inacessible for harvesting in the water near Eudora. The one I was previously able to harvest was right behind Eudora and to the right side of the water if you are facing the water. The one I found last night was on the left back side of the water and cannot be reached. I did try from the ramp going up beside the water, but did not try from the bridge above the water.