An underwater slate is used to communicate information or as a memory aid.
Divers carry knives today not to protect themselves against sea creatures but to be able to free themselves or others in case of entanglement.
Spare Parts Box
This can contain, for example, spare O-rings, fin straps, basic tools, spare oxygen cell, battery, mask strap, etc.
Mesh dive bags are very useful to carry mask, snorkel, fins, hoodie, drinking water, camera, cap, or any accessory required for the dive. They make it easy to rinse everything in one go at the dive site after your dive.
There are also larger dive bags designed for traveling with your dive gear. These may have many compartments, wheels, wet-gear pockets. They may come in various sizes and styles.