issue for generations. The history is the felt soles took over the market for a long long time and they do work very well and stick to slime covered rock extremely well. But as is pointed out above, in the last few years felt soles have been banned in many places (including Vermont I know) and so they are not even being manufactured anymore. What has replaced them are soles made of special "rubber" formulations that are soft and stick on rock reasonably well. Also, there are souls of this new material with studs, not unlike you see in studded snow tires. If you look around at good fly fishing stores/websites you will find various options.
Personally when Kayaking I wear the NRS Neoprene booties with rubber soles. There are pretty good. But when I am canoeing I wear Tingley rubbers - available in different heights. On trips I wear the ones that come up to just below the knee. These are cheap. They last several seasons and the grip surprisingly well. They are easy to slip on and off over sneakers or whatever you ordinarily wear. I use them at home in the winter when I go out in snow to shovel or snow blow. They work great for that too.
Just some ideas.
Touring Kayak Paddles
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