This is on level 3 after you
You have a group of enemies to kill and then after you your way towards the primary marker, it will be about 200 meters away you will see the yellow floating inner tube on your left. It is right after the dialogue with Romeo still being stuck in the elevator and Claire saying suck it up and you say she is growing on you, "oh bollucks." Then should see it. You need to hit
to "use" and it will start floating down the river. JUMP ON IT!!! Fun time. I got knocked off by a tree and then a poll, unsuccessfully trying to jump over either. Don't worry, just catch up to the inner tube and jump on, there is a nice long area after those two obstacles that give you plenty of time to ride the thing for the achievement. You need 20 seconds, it does not seem to be consecutively. If you are doing REALLY bad you may want to redo your checkpoint sooner than later, because once the tube stops it's a new check point and you need to restart the level to try again. But like I said, it's not too hard at all, and if you should get this fairly easily.