10-12 miles from the shore is quite a way in a small boat. I think you are kidding yourself on if you think anyone on shore is going to see a flare at that distance and respond to it, unless it’s dark, good visibility, and there’s something like a coastwatch station pointing your way. A rocket flare is visible for something like 40s, but at 10 miles it will be much less, if someone who knows happens to be looking.
You might be unlucky with your bay, but usually you can get more than a few miles range, because the CG mast has some serious height. All local shipping will also hear a mayday. I do notice an improvement with my fixed vhf though, even mounted on a pole about 2m above sea level.
If I was going that distance alone on a regular basis then I think a PLB would definitely be on my shopping list.
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