Very good points made by all. Somehow I feel wrong if I allow a thread like this go by without giving, based on my own experiences with various training programs, a thumbs up to CrossFit (as others have done as well).
Nothing will prevent you from being stiff after your first day back on the water. Nothing. But if the relevant muscles are in shape (via CrossFit or other) it’s days rather than weeks of stiffness. But more than that, you’ll have the strength to carry you through the season performing better, and with a lower risk of injury.
Kettlebells, core, pull ups, Olympic lifts, deadlifts, high intensity intervals, farmer carries, sled pushes all translate to slalom well, and incidentally are all emphasized in CrossFit. If you have the time and money, it’s way more enjoyable to do these in a competitive group setting, than making your own workouts up and then throwing weights around in your garage/local gym by yourself. Slalomers seem to be the type of people who thrive obsessively in competitive environments so I reckon many would enjoy this type of exercise setting
Apologies for sounding like an infomercial :\