I won't claim to be the expert of buoyancy but I will offer my 2 cents...
I used to think the same way - a diver needs to be able to maintain their depth without any reference. I thought that I was a pretty sucky diver as I could not maintain my depth without a reference. Then it dawned on me - maybe its not that I need to be able to maintain depth without a reference but rather, I need to know what references I have at my disposal.
In recreational diving, you almost always have a visual reference. These references include:
- the reef - during the dive, you are usually looking at the reef. The reef gives you a visual reference
- anchor line - if you are diving from a boat, the anchor line can serve as a visual reference. Note that if there is a lot of swell, the boat will move the anchorline up down and therefore it becomes less reliable
- spool with your SMB - if you are ascending and the boat is not anchored, you will have to deploy your SMB. You can lock your spool such that there is tension on the line while you are at your desired depth. This actually works great for you as you can feel when you are ascending or descending just by holding the spool. It is also works great as the spool will give your buddy a visual reference.
- computer - ideally, you will have other reliable references for depth. But if you don't, for example, if you are travelling mid water or you are ascending on an anchor line and the anchor line is moving a lot due to swell, you can use your computer's depth gauge. Just keep in mind that you cannot fixate on your computer. You need to be diligent to maintain situational awareness of other things beyond your current depth.
- your buddy - in an unfortunate case where you lose your mask during a dive and no backup mask is available, your vision will be impaired. In this case, your buddy can serve as your touch reference for your depth
- if using one, your drysuit - this one is kind of imperfect. At least for me. But it will tell you - if you have adjusted your drysuit for the current depth you want to be at, you will feel it get tighter if you descend and get loose if you ascend. For me, by the time I know, I usually moved a couple of feet already. Anyway, it is an indicator all the same.
I can't think of a situation where you have no reference for depth when doing recreational dives. The only one I can conjure up is if you purposefully dive when the environment has almost zero vis.