The following are instruments that are used for scuba diving.
Underwater timing devices are watches that are waterproof up to a predetermined pressure. Look at the manufacturer’s specification regarding the absolute pressure rating, and ask if it can be used for scuba diving.
Cylinder Pressure Gauge
Submersible pressure gauges (SPGs) show, either in digital or analog form, the remaining pressure in the cylinder. Pressure gauges can be mechanical or digital.
Depth gauges work similar to pressure gauges and are based on the same technologies. There are digital ones with transducers and mechanical ones.
An underwater compass works just like a land unit, except that it must be waterproof. Select one that has a rotating bezel and that is oil-filled to dampen the movements of the needle. You will be learning basic compass navigation in your open water dive.
You should have your own compass when you learn navigation in other diving training programs.
It is not necessary to have a thermometer, but it is nice to know the water temperature in order to properly dress and plan ahead. Thermometers may be separate instruments, but more often, they are built into one of the other instruments, such as a personal dive computer (PDC) or SPG.