Thank you for pointing this direction. I was fishing in Vedder river for 7 hours yesterday and there wasn't even a bite. I've been wondering what went wrong and now I think this could be it. I've been considering locations, weather, time, tidal, presentation depth, jig colour, etc but I haven't doubted the floating jig method. Since I have succeeded once and there is even a catagory of "floating jigs" in many tackle stores, I would trust this to be an efficient method. But now on second thought it may not be the case.
What they call 'float jigs' are really just tied on lighter heads, and you can add them to your regular setup without having to change the weight or float. They might still run a few grams, so if you have your lead weight is close to the buoyancy limit of the float (for max sensitivity), then even a float jig may bury it. They also tend to be smaller, and not have the huge profile some twitching jigs have.
FWIW, float jigs are commonly used for chum (and pinks earlier in the season). For some reason, chum really take dead-drifted jigs, while coho 'usually' do not. Not 100% sure why they dont like drifted jigs, but coho like to chase stuff, so active presentations like spoons, spinners, jigs, flies, really benefit from some motion to trigger that response. In the right conditions, sometimes you'll see coho follow something like a spinner quite a long ways before biting or losing interest.
Passive presentations like roe, beads, wool, will be readily taken dead drifted. There must be something about the color, size, or shape of those things that triggers that response. (and scent, in the case of roe). If you dont have access to roe (or dont want to deal with the mess) consider adding some beads to your arsenal. They can be surprisingly effective, and so easy to use.