It was a road tax, it was renamed and couched in environmental terms solely in order to justify a hike in tax, sorry, duty, with the intention of not losing too many votes.
All road users all vehicles should pay fir the upkeep of the infrastructure.
Most funding today from river boat users goes towards keeping flood defences sound to protect housing, or maintaining riverside footpaths and bridges for the public to use. It used to be heavily subsidised by government, for the benefit of the nation, since it was recognised that not only boaters benefit from waterways. Now funding us so reduced that boaters pay exorbitantly for the lot, and fishermen hate the boats that go up and down but have paid for their access. Try getting me started on river licences!!