There are two giant squids that can be found near the end of chapter 4. Close to the Blue Portal that you enter to advance the story is a meditation statue. If you sit on it and meditate, you can find a Giant Squid. Swim close to it and press
to ride it unlocking the achievement. You don't need to use the meditation statue. However, there are currents in the area that can be distracting. So, there is some advantage of spotting your target while sitting serenely and going directly to it. On the other hand, you can find the Giant Squid just by swimming around.
The other Giant Squid is a little easier. Had I not already unlocked the achievement, this would have been my preference. After you enter the Blue Portal that advances the story but before you enter the door it opens, look around in the immediate vicinity. There is a giant squid swimming about. It is easier to spot than the one outside the Blue Portal and you are not bothered by distracting currents. Swim close to it and press
to ride it. The achievement will unlock unless it has popped previously.