CNF (constant weight without fins) is a tricky discipline, especially in the winter, since the colder the water is, the thicker wetsuit you need to wear, which means more weights to wear and bigger buoyancy change = harder on the first part of the descent, and also harder on the deepest part of the ascent. Some freedivers don't want to hear about training CNF in these conditions, while some others use this as a training tool: in fact you can argue that if you do (shallower) dives under hard cond

8 Tips for Beginner Freedivers