New divers often have difficulty staying with a buddy during descents and ascents. These are critical phases of the dive during which the probability of buddy separation is heightened and possibility of injury is high. Avoiding this situation in the first place comes down to an issue of buoyancy control.
As your buoyancy control improves, you'll have less of a problem with buddy separation during these phases of the dive. It's generally considered good practice to stay within an arm's length of your buddy during initial descent/ascent.
Descending/ascending under control means being able to hold one's position in the water column at any given moment. I try to keep finning at a minimum so that I don't tire myself out.
During a descent, I aim for neutral buoyancy and then exhale a little to begin to descend slowly. Once downward momentum begins, I add just a little air to my BCD or drysuit...and then aim to become neutrally buoyant again. The entire time I'm watching my depth gauge and my buddy.
During an ascent, I essentially do the same thing in reverse. From a neutrally buoyant position, I inhale slightly more deeply. Once upward momentum begins, I exhaust just a little air from my BCD or drysuit...and then aim for nuetral buoyancy again. The entire time I'm watching my depth gauge and my buddy.
If my buddy begins to get away from me during a descent, I can grab onto his rig and help slow his descent. If I can't grab him, then I'll follow his bubble stream down at a controlled pace. It makes no sense to be crashing into my buddy while doing my best impression of a dirt dart.
If my buddy begins to get away from me during an ascent, I can also grab onto his rig and help slow his descent. If I can't grab him, then I'll follow him up at a controlled pace. In most cases, I'll omit safety stops to make sure that my buddy is OK on the surface.
Particularly with uncontrolled ascents, I find that making noise to get my buddy's attention does no good. He's already headed to the surface. He knows that. I know that. We both know what the other should be doing at that point.
For a buddy to drop like a rock indicates they are massively over weighted.
Or that the diver is unable, for whatever reason, to add gas to his BCD or drysuit. I've seen this happen with a buddy whose BCD inflater hose popped off of the power inflater assembly shortly after leaving the surface during initial descent. Once his strong downward momentum had already been established, it was difficult for him to add enough gas to his BCD by oral inflation to arrest the descent. Fortunately, we chose to descend over a sandy bottom that day. It could have been ugly if we were descending in an area where the bottom was beyond recreational depths. This would be an excellent way to damage one's ears if the descent outpaced ear equalization.
I agree that proper weighting is important. After fully emptying his BCD at the surface with a full tank, he should never be more negatively buoyant that the weight of his gas. Attention should also be paid to pre-dive gear/buddy checks to prevent an issue such as the inflater hose from popping off. A slow, deliberate descent gives the diver two additional safety options: (1) finning towards the surface and (2) getting a buddy to intervene.
