You will earn credits for finishing better than 4th in all races and events, and even in multiplayer and private matches. You will get a good amount for each place. Using the expert boats speeds up the winnings considerably as you receive more.
Novice Boats - 1.)500
2.)375
3.)250
Pro Boats - 1.)1000
2.)750
3.)500
Expert Boats - 1.)2000
2.)1500
3.)1000
Easier to do in private match and then you can work towards this acheivement too
Just make sure whoever is winning the race uses an expert boat to speed up the process!