Simply win podiums on races, 3rd will do, once you have enough money you can then upgrade your hydra. On the race select screen, across the top left you'll see the name and Index of your current jet. Navigate up to it with the left stick, and click A on it.
Select Upgrade, and a list of upgradeable attributes will come up. Highlight one of them, and hit Up on your left stick. Once the Buy button is highlighted, press A. Achievement unlocked! You also have an upgraded water-bike! Keep doing this, once you have a ton of upgrades you'll notice the cheating AI is much easier to beat!
Once you fully upgrade your water-bike you'll also unlock Tricked Out for 30G's :)