First 2 Races are in Main Canal (city).
Second 2 Races are in Slaughter House (you have to enter one of them for story progression, you can't get here until you get the jump ability for your hovercraft, item that is 15 pearls in Mamangos).
Overall these are very easy to get first in, make sure to come equipped with boosts as they will help a lot, the races aren't very long, always try to get onto the boost arrows on the tracks too, once you have done first place in all 4 races you'll get the achievement plus 4 pearls for each race's victory.