Unsure what I'm doing wrong, the mod is enabled and up to date but I can't place any walls or gates on shallow or deep water, trying to use stone walls and stone gates. Also tried others just to test. Factorio is up to date.

Sorry for the late response!
1. What mods are you using?
2. This mod only lets you place walls on shallow water, water tiles that your character can already walk over (which is different from regular water). Does this problem apply to 'shallow-water' (best tested in the map editor or in the 'Alien Biomes' mod)?

So after some more testing I found the problem, not sure which mod is doing this but somehow, regular (non-walkable) water was replaced with Deep Water and shallow water (walkable) was replaced with regular "water" that can be walked on.

The only major mods that add tiles I have installed are like Factorio+, Alien Biomes, & Asphault Roads. The rest of the mods are mostly QOL or non-generative.

So I guess the problem is the water that appears to be shallow water and can be walked on is really regular "water" ... strange!! easiest solution will be going thru /editor to swap water with shallow water.

There is a possibility this worldgen is being caused by the Cargo Ships mod.