Steer your kayak down the river, avoiding the rocks to stay afloat as long as you can. Every in-game second you stay afloat earns 1 "money" (i.e.point). Use your money to upgrade your kayak: either buy a different kayak, or buy upgrades for your current kayak.
You can upgrade your handling, speed, and stability. Handling refers to the speed with which the kayak moves left and right; improved handling makes it easier to dodge the rocks. Speed refers to both the speed of your kayak, and the speed with which the in-game clock ticks. If you buy more speed, your kayak will move slightly faster; however, the in-game seconds tick faster earning you more money. Stability relates to the kayak's performance in the rapids; the more stability your kayak has, the slower you go through the rapids thus giving you more control in trying to avoid the rocks.
The alternative kayaks and the upgrades are bought from the boat house shop.
When you quit the game, your money total, upgrades, and kayak are saved, ready for when you want to play again.
As you progress down the river, the challenge increases as the rocks appear more frequently. If you quit the game and come back to it later, your kayak journey resumes at the point in the river where you had quit.
Please note that I have lost the editable version of the game and so I can not update Kayak Dodge.