Regarding diapers: supermarket brands are cheapies and can leak (e.g. Tena, Depends, etc.)

Instead you'll want to take yourself to a pharmacy or care store to seek out a proper continence pad, yes they cost slightly more, but they work. These will not leak as you move about in the dive as the moisture is converted into a dry gel via super absorbent polymers. They feel dry to the touch and as a bonus will balance the pH as to not interfere with the skin's acid mantle. There is no threat of breaching the capacity as absorbencies can be in the litres.