Players racing status is set to "Racing" by default. You can double check your status by going to Settings:
1. Tap the gear icon in the upper-left corner of the screen to open Settings.
2. Go to Additional Settings.

If there's a check mark next to Participate in Regatta, it means that your participation in seasonal regattas is confirmed. If there is no check mark, and if you do not confirm your participation before the start of the next regatta, you won't be able to take part in that race, and you will have to wait for the next one.

Your co-op enters regattas as soon as any one of its members confirms their participation, but the competition only starts when at least one of the members, irrespective of their rank in the co-op, picks a task.

ATTENTION: You don't need to confirm your participation in interseasonal regattas—you can join them on any day while they are being held.

To participate, simply tap the Pier to open the regatta window and pick a task.

Learn more about situations that prevent you from the participation in regattas in the following article: Why can't I participate in regattas?