There was a big study on brain aging. David Snowdon followed nuns for many years.
en.wikipedia.org/wiki/Nun_Study
His conclusion was good outlook in life, exercise and density in thought is good prevention on Alzheimer.
Of course, nuns don't go surfsking hahaha, but people interpreted that as use it or loose it argument. The more you use your brain and the deeper you use your brain, the better.
Surfski helps with exercise, use of brain in balancing perhaps and if you are enjoying the sport, you have good outlook on life.
If you have to study the waves and recognize pattern, that is a lot of thought to process. You have to balance on top of that. David looked for density of thoughts on nun's essay, so I would say surfski downwind is similar.