Hi Scott

How are you getting the scaffold to the structure? access and egress and carrying scaffold long distances
Recovery of persons from the water (still or fast flowing water) if they fall in? inflatable lifejackets, a manned rescue boat, liferings + hand line
Hypothermia can kill in a very short time if prompt action is not taken
Pollution of the waterway from overspray, dropped tins of paint, diesel and oil spills and scaffolding materials - are there other waterway users that this can fall on?
Also weils disease and other bio hazards (Dead animals, Birds + their droppings etc) cn present a risk

I hope this helps