Years ago, I used to race flatwater kayaks. I remember doing a ten mile race in a lake (no current assist) in ~1:20 - about 8 min/mile. The only problem is if you’re not pretty skilled with a “down river” or “wild water” racing kayak, they’re a bear to handle. Think “only stable while upside down”. I believe the same holds true for surf skis as well…
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I used to have a surf ski – got it as a teenager and had it through college. I had no problems with stability on it, but whenever my dad or another adult male tried to use it over they went immediately. Must have been their higher center of gravity.