Despite some debate over the best way to manage a search line, the consensus is attaching a diver to a search line by a harness or BCD.
Mark Philips authored an article for PSDiver Magazine in which he described how to secure the diver by a tether line. This safety line is the most secure path back to the surfaces and frees the hands of the diver for search or handling tools.
He insists that in zero visibility the diver should not use a “quick release” mechanism to attach the diver to the harness. Should a diver who is not fixed to a search line come entangled and drop the search line, there is no way the backup diver to follow the line to aid the diver. Remember that a backup diver should assist the diver in less than a minute.
If the diver needs to get off the line, he can cut the line with many of the cutting devices that should be carrying. At a minimum, a diver should have two (2) pieces of “cutting devices”, each one of them should be easily accessible by both hands at any time during the dive. He expressively said: ” in zero visibility, I always want to find my diver ON THE LINE”. For him locking carabiners is the best choice.
The attachment point could be on a harness or BCD (for me, it is better to use a separate harness form the BCD and use it as attachment point), because this point should be hard enough to take any kind of abuse. Then the diver will have a “hand loop” to hold on while he is getting directions.
The diver will have a slightly slack underneath when switch sides, this will not represent a problem. The knot where the hand loop is tied will always remain fixed, this knot is called ZERO KNOT. This point is used for measurement as the line also should have colored zip ties attached at already measured distances; for example, the recorder will write something like: “red zip tie to a large pole on the west of the walking concrete path of the beach”.
One thing that was not discussed is the ability of the diver to attach the line to an object and use a tag line/ contingency line to return to the surface. A tag line also could be used by a backup- diver to get to the primary diver to help him.
Read the entire article here: Anatomy of a search line