This seems pretty simple but in order to complete a race you must first buy a boat. The boat shop is located in Sausalto, Marin, near the top of the map. You can fast travel directly there if that's your style. The cheapest boat is called Ocean Breezeway and costs $3000. While you can buy a different one if you want, there's not much point.
After you have your boat, face towards the water and head to your right. You should see a man in a blue jacket leaning on a railing. Press Y to talk to him and the sailboat races will unlock.. Turn around and there should be a guy standing around in a suit. Talk to him, select whatever race takes your fancy and complete it, making sure to follow the arrows on the buoys so the checkpoints count. The achievement should unlock as you cross the finish line.