Keys are items that are featured only in the arcade version of Hydro Thunder. They are needed to unlock the Medium, Hard and Bonus tracks, as well as the Medium and Hard boats.
The keys are designed after vintage lever tumbler lock keys, with the bow shaped after the game's logo. They come in red, yellow and blue.
When you complete a race in the Hydro Thunder arcade game, you earn a key out of predetermined number needed to unlock the track and/or boat in the game. Getting enough of the needed keys would give you access to the track and/or boat. The keys for unlocking the Medium and Hard tracks not only give you access to those tracks, but also allow the use of the Medium and Hard boats. From there, more keys can be collected to unlock the Bonus tracks. When you collect the required number of keys, you would see an animation where the lock representing the locked state of a track is unlocked. However, if you did not win a free game after completing a race and when the Continue countdown reaches 0, the keys collected in the process of unlocking the track would be null and voided, giving you the irritation of having to start over again.
However, the game can be set so that every track and boat (except for the Bonus boats, which are usable only with a cheat code using the machine's pilot view button), are available for play, eliminating the need for having to complete race after race in order to unlock them.