We'll do this job in June when it is sunny

Perhaps not.
There were two guys doing this up to their waists in what must be cold water. I guess they are clearing the river bed of weed to make sure it does not get clogged up. The Itchen is also noted for trout and they need gravel/chalk to lay their eggs in.
Apparently this weed clearance is always done in June. No effort is made to collect the weed that is just left to float away. I suppose it becomes someone else's problem. It might explain why the St Catherine's Hill Lock looks rank at the moment (it smells as bad as it looks).