Underwater Slate or Wetnotes
An underwater slate or wetnotes (a waterproof notebook) are used to communicate information or as a memory aid. An inexpensive and useful addition to your dive locker, a slate or a notebook should be stored carefully and carried in a spot where you can get at it easily.
Knives and other cutting devices come in many shapes, sizes, and prices. Many divers carry more than one and carry backups on their bodies to make sure the knives can be easily deployed in the unlikely event of diver entanglement.
Spare Parts Box
This can contain, for example, spare O-rings, fin straps, basic tools, spare oxygen cell, battery, mask strap, etc. You can equip a spare parts box (also known as a save-a-dive kit) for a small price, and it can literally save a dive or save the day. At the other end of the scale, some divers carry a wealth of spare parts including wrenches, oxygen-compatible lubs, a full tool kit, spare hoses, regulator service kits, and esoteric items such as voltmeters!
If you are traveling to remote dive sites, plan with your buddies to spread the load and expense of traveling with at least a good list of spares.
Mesh dive bags are very useful to carry your mask, snorkel, fins, hoodie, drinking water (in a reusable container), camera, cap, coral-friendly sunscreen, 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, and wet-gear pockets, and they can come in various sizes, styles, and prices.