You might increase your chances by using a faster sink-tip line, fast-sinking poly leaders or tungsten beads, which will enable your fly to reach slightly greater depths in the heavier water, but it is important to resist the urge to strike at every bump or tug, as your chances of snagging fish will increase as well. Using "circle hooks" will also help to redue the chances of snagging fish.
I tend to use a medium-paced, erratic hand-twist for situations such as this, and employ a strip-set once I've felt the weight of the fish. This retrieval method allows "followers" the opportunity for multiple strikes when the fly pattern has a longer tail or if they miss their target - while avoiding the flesh of salmon that the hook might have inadvertently bumped into.
Another proven tactic is to "high-stick" with heavily-weighted egg patterns or peach-coloured Czech Nymphs. I have caught many Coho in pocket-water and pools using this technique, where 'conventional' methods simply aren't as effective.
If you're fairly new to Coho fishing, I'd personally recommend focusing on the type of water that is more conducive to fly fishing (frog-water, shallow pools and sloughs), which is generally much easier and offers instant gratification that builds confidence for newbies and novice anglers. The key is to make any form of angling a fun and enjoyable experience, as a happy and confident angler is a successful one...
Cheers,
Todd
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