How do I claim my reward for successful participation in a seasonal regatta?

After the end of the regatta and before the start of a new one, you can go to the regatta tab and tap on the reward button for distance covered (the third button in the lower right corner). There you will see the number of cards earned for buoys reached in the regatta. Each card comes with a reward. Rewards are randomly generated and you can select and change cards with unwanted rewards three times for free.