I am not doing this math under water or before the dive, just turn the bezel half way round to go back or 1/4 way left to go right.
That is probably a little tip the instructor will throw in but the correct way is to do the math because that works in all situations. It is easy to navigate a square without math. When you take the navigation specialty your instructor may have you navigate a triangle and then a right triangle before moving on to the more complex skills and knowing how to do the math helps greatly.
The point is not that you are learning how to navigate a square. You are learning how to navigate and demonstrating those new skills by doing something simple like a square.
OWD is just an introduction that teaches the basics. The skills can get much more complex and build on skills you learned in earlier training.