It is ? Wow. There should be sufficient highways/bridges in the country's transport infrastructure already paid for, and maintained by, public money that constant ferry ticket costs don't come in to it. Shouldn't be any separate charging schemes, especially when the drivers are just used as cash cows.
My Dad could remember when the boats were paddle steamers when he was a kid (before they brought in the end loading diesels in the 60's) and you could go and watch the engines working. No Health and Safety then!!
Spent many a time as a kid on the paddle steamer versions going backwards and forwards. Watching all the machinery work, highly polished brass and copper. All the sounds and smells as well.
Long may it remain free.
Do definitely need another road crossing between North and South, at the moment it's either Dartford or Blackwall Tunnel which is quite a gap