I don't know of any diving courses that teach this. I haven't seen it in any syllabus. I'm talking about diver identification underwater. How do you recognize your buddies and fellow divers while you're all underwater? You can't talk, obviously, and you can't hear very well. You can hear sounds but they're all muffled and you can't locate the direction of the sound source. One of the most dreaded sounds is the drone of a ship or the buzz of an outboard motor overhead. You don't know where it is coming from, how near it is, whether it is headed your direction, whether it's going to pass over you, whether it's going to hit you as you ascend... it's definitely not a nice feeling to be hit by a boat propeller. It's like, see you in heaven, and make sure you get there (but it's already too late to do anything about it at that time anyway).
Anyway, I was just making the point that it's really difficult to communicate by sound, and impossible to speak unless you have those funky underwater radio communication systems. It doesn't help that tank bangers and shakers sound the same. So when you hear a sound and look around, how do you recognize your buddies? It's important to know who is who so that you know where your buddies are, if anyone is lost, or just so that you can "talk" to the right person.
Thus divers rely on visual communication such as hand signs. And to identify my buddies, I look for visual cues and identification features as follows:
The #1 telltale sign of a diver is the bubbles, which can be seen from far away. This is the first step, to identify that a diver is present. Bubbles reflect light, and stand out as silvery trails rising upward against the backdrop of blue or green or whatever colour the water is. The only problem is when visibility is bad, then you have to really strain your eyes to spot the bubbles.
It's easy to spot a diver once you trace the source of bubbles. Even if there is a visual obstruction in between the other diver and you, such as a boulder or coral outcrop, you can still see the bubbles rising above the obstruction. Another thing about bubbles is that they are the only way to spot a diver from above water, unless the water is so clear that you can see to the bottom.
Bubbles tell you that a diver is there, but it's almost impossible to tell who that diver is, unless you know that diver's bubble pattern! Like I said, it's almost impossible, so to identify a diver you need more definitive clues...
Scuba tanks are big and usually bright yellow, or shiny aluminium. So they're really useful for spotting a diver from afar, such as in situations when I have lagged behind the group to play with a cuttlefish and I need to locate my buddies to regroup. I know, diving mantra #1 is to always dive with a buddy, but sometimes the buddy wanders away, or I linger while the buddy moves on. Ah, well. Just look for the bright, shiny tank. Sometimes the dive operator has different colour tanks, so I can identify who's who by matching the tank to the diver, but I would need to remember before the dive who is using which type of tank. For example, Diver A = yellow tank, Diver B = silver tank, Diver C = yellow tank with badly peeled off paint, Diver D = silver tank with sticker, etc. The problem is, usually the tanks are changed for each dive, so you have to recalibrate the ID before a dive. But if the tanks are all the same colour, then, too bad.
Fins are really useful identification clues. As there are many brands and models of fins on the market, usually each diver would have a different type. Fins are also very visible from far especially as many types of fins are brightly coloured and have distinctive patterns, except the black ones. I find this one of the most useful identifying features. I usually look for the fins first. A positive ID of the fins and wetsuit (below) is usually enough to identify who's who.
The wetsuit is useful for identifying a diver as wetsuits come in different patterns, designs and colours. The designs on the legs and arms are the most visible, so I look out for them too while looking at the fins. Some wetsuits are full length, some are shorties. Wetsuits are visible from far as they cover most of the body of the diver. But if everyone is wearing similar wetsuits, e.g. they're all rented from the same operator, or if everyone is wearing black, then you'll have to look for other ID features. Some divers wear beanies over their head. Some wear sleeveless vests. Some divers don't wear wetsuits.
5. BC & Regulator
It's easy to identify a diver based on the buoyancy compensator (BC) a.k.a. buoyancy compensating device (BCD). BC's are visible from far, and have different designs, patterns and colours. The problem is when divers have the same model of BC, or similar looking BCs, as is often the case with rented equipment.
Another identifying feature is the regulator, but this is not so obvious because it is rather small. You could also look at the arrangement of the hoses, e.g. how the octopus is attached, the colour of the octopus hose (yellow or black?), whether the pressure gauge is clipped on or dangling below and leaving a trail of destruction as it drags over the corals. (In such cases you should swim over and politely help the diver to tuck or clip the gauge to their BC. If he/she refuses, turn off his/her air. Haha. Just kidding.)
6. Mask / Snorkel
This is a bit harder to use as an identifying feature because you can't really see the mask until up close, in which case you would already be able to see the diver. But if the diver has a snorkel attached to the mask, then it's a pretty conspicuous item visible from far. It also helps that snorkels tend to be brightly coloured.
7. Other Gear
Some divers wear gloves. Many wear dive computers nowadays. You can also look for other clipped-on gear, like the reel, or sausage, or torchlight, or pointer, or any other device. A photographer will be carrying a camera, so you know who he/she is. If there is a whole group of photographers, then you gotta figure out which camera belongs to whom. It's pretty easy as cameras and their flashguns are big, chunky items. Yea, you know for sure that a photographer is around when you see a flash firing.
8. Personal Traits and Habits
What I mean is, each diver has his/her own characteristic diving traits and habits. It's like, on land you recognize a person's body shape, or a person's gait, or the way he sits, or the way she folds her arms, or the usual hangouts of a person. Same thing underwater. Look at the diver's body shape (I shall not comment on this, except to say that divers look different from fish). Each diver also has his/her own style of diving. Some divers like to zip around here and there. Some like to move slowly. Some look like they're lost. Some look like they know where they're going. Some lead. Some just follow. Some hover horizontally. Some hover vertically (I notice that it is generally easier for ladies to maintain a horizontal hovering position while motionless). Some like to swim close to the bottom, while yet others like to hover above everyone else, like a helicopter mothership. Photographers will usually be glued to one spot for a while, or be crowding around one poor little creature at the bottom, or nowhere to be seen. Photographers also tend to get left behind, so if someone is missing, it's probably the photographer. Thus you can tell who is who by observing where they are. You can also watch out for a diver's fin kick pattern. Some fin hard, some fin slow. Some use the frog-style kick, some use the up-down kick. Usually beginners will use their hands and arms a lot, but as you get more experienced you hardly use your arms.
9. Anything Else
Whatever you can find. Anything unique.
In short, use a combination of the above to differentiate and identify each diver. Happy diving, and appreciate your uniqueness. :)