Is weed aways an obstruction ?

As for the river  you could see the bottom maybe 20ft out from the river banks.. I've never seen the bottom of the river before, except in the depths of winter.. The weed and lilies think this is wonderful, clean water and a hundred years of farming fertiliser run off on the river bottom to grow on..

There are areas on the river, we used to sail into, but now have to call for water to clear it, as it's lily and weed infested...  No one has questioned this it's just been accepted as the right thing to do.

There is also the possibility of Hovers, the old name for a mass of weed / reed / grass / peat that has broken away from the bank.. they're not common but at least you can see them easily drifting down the river. 

Old broads classes had Spoon rudders Hydrodynamically not very good, but they catch a lot less weed than a modern spade rudder.

'Headlines'      Editor of prestigious yachting publication searching for 'weed'.

What are peoples actual experiences ? 

I’ve been sailing for 35 years always on narrow rivers or small lakes. I’ve known areas of weed near the bank marked off in some way as a obstruction or marks moved so they are out of a weedy area, but I’ve never had ever had anyone call for water to tack because of floating weed .