I used to train in the dark (marathon, river race, slalom) all the time. You get used to it. You don’t move that fast so a simple head torch will do unless you are on a busy river. Scares the life out of you when you hit a duck and it flies up though.
I used to spent an unhealthy amount of time at Teeside barrage. It’s lit but not very well so it was a bit like paddling in the dark.
I’ve got off rivers in the dark before, never intentionally, normally because it all went a bit wrong somewhere. You can add a grade to any normal river if you don’t have lights 🙂
I used to go to Stanley Embankment quite a lot, it’s a surf wave under a bridge in Wales. It was fine in the eddy but pitch black if you got on the wave. Just had glow sticks on the helmet so people could see you if you swam.
Finally on the more sedate side I did some night paddling on a trip in Canada because we were trying to cross a large but shallow lake. It was very rough during the day but calmed at night. It was pretty close to the arctic circle so it was night but never got any darker than dusk and we had a 6 hour sunset. It was very eerie.