Plus the visibility is not good so you can’t see far away. I observed the other divers, and they don’t use any paraphernalia to navigate. What is the secret? (bold added)
Ah.... but they DO. You're just not seeing it.
Navigation becomes a lot easier once you start using *all* of the cues available to you. The navigation section of your AOW course is, unfortunately, a very poor introduction into what you really need in order to navigate confidently.
So what cues can be used?
1) time (tempo and air-supply/planning)
2) depth
3) compass direction
4) familiar objects and/or cues on the bottom
5) current / tide
6) light and shadow
7) sometimes sounds like anchor chains.
8) I'm sure some people have other cues they use too...
I'll deal with all of these topics briefly. If you have any questions, ask them.
1) Time, tempo and air-planning: This is your biggest tool for navigation much of the time. Simply put, in many cases if you swim for (say) 20 min in one direction and then turn around and swim at about the same tempo for 20 min in the other direction then you should be about where you started. This assumes that there are no heavy currents and that you have enough cues to know that you're swimming in a reciprocal course. I would say in most cases where you see divers navigate accurately but without any "paraphernalia" then what you're missing is that they picked their "turn around" point to correspond to a certain time or air-pressure.
2) Depth: The bottom is very seldom completely flat. If it is, then you need a compass. However, almost everywhere you dive there will be some depth change as you go even if it's only a metre or two. Even that amount of depth change is enough that you can pick a certain depth and swim along that (virtual) "depth line" from point A to point B. In your case, for example, you could descend from the boat to the bottom and record/remember the depth at the anchor. Then start swimming in the "out" direction. At some point you turn around (time/air cue) and ascend to your "depth line" as you swim back so you're at the depth where the anchor should be at about the time (or shortly before) that you expect to see it. I would say that probably 90% of "natural" navigation in practice is done by watching "time" and "depth". Oddly, that's not stressed the book.
3) Compass: For straight forward dives you only ever really need a compass if the bottom is completely flat or visibility is so horrendous that you can't see the slope of the bottom. Having said that, even if conditions are good you should always have a compass with you so you can answer questions like "which way am I swimming", "did I really turn all the way around" or "which way is the shore". However, in terms of navigation, the compass is generally a much less useful instrument than "time" or "depth". Also, when using a compass in a team, it works best when one diver generally controls "direction" and the other diver controls "tempo" and "depth". Assign or "delegate" tasks for best results.
4) Bottom cues: This can be anything that doesn't move. A few of them are mentioned in your AOW book but generally I would look for something really obvious like a foreign object (ie discarded junk, a net etc.). Also "surface" cues can be handy too if your viz is good. Perhaps you can see the boat or an anchored bouy. Alternatively, you can create your own cues by disturbing the bottom (if appropriate). In any event, the more you dive at a given site, the more likely you are to find things you can use for cues.
5) Current and tide: These will give you clear indicators of direction and can be either a help or a hindrance to navigation depending on strength and direction. My advice if you're unsure about navigating a site is to eliminate this effect by diving a slack tide until you "get your bearings"
6) If viz is good you can use light and shadow to determine direction. Sometimes, for example, you can see the position of the sun from your dive depth which can tell you which way you're going if you're not sure and/or don't have a compass.
7) Sounds: We're trained that sounds are a poor navigation aid because you can't determine direction from them. However, some sounds can be very helpful. A rattling anchor chain is one of those. If your boat uses an anchor you may be able to hear it from a distance. You may not be sure of direction, but if you listen carefully, you WILL be able to hear "volume" (ie how loud it is). This can sometimes help you decide "if you're there yet", or even "if you have gone too far".
8) Like I said, I'm sure that other people have cues taht they use that I haven't mentioned. Also, just to recap, the most reliable and accurate tools you have for navigation are "time" and "depth" provided your dive site isn't completely flat and you are able to swim "out" and "back" with about the same tempo. One last point to make is that awareness of your surroundings is very important to accurate navigation and it may require you (often does) to "slow down" and spend more time consciously observing your surroundings. A bit of advanced planning and having some kind of a mental map of your dive site doens't hurt either....
Hope that helps.
R..