Some water sports providers offer insurance on the day as part of the package. But check the terms carefully to make sure the cover is adequate and look out for any exclusions. Also, before you sign any waivers, make sure it won’t invalidate your right to claim.
If you already have a plan of what sports you want to do, it may make more sense to arrange cover before you go. That will allow you to compare your options and find the best deal for you.