I haven’t fished much flounder in the San Juan’s specifically. You don’t need to go that far from Everett where we’re moored since the sound is more or less covered with them. You’ll catch lots of Pacific Sand Dabs, Southern Rock Sole, occasionally a Starry Flounder, English Sole, or Rock Sole. Once I think I caught a C-O Sole.
In terms of gear, you don’t need to over think it. A lightweight spinning reel and a lightweight spinning rod is all you need. I like Ugly Stick because the tip is sensitive, but these fish aren’t picky. For years one of my rods had the tip to the second eyelet broken off by my sister when she tried to “set the hook” and bonked it in the Bimini top. Pro-tip, you don’t really need to set the hook. Most of the time, flounder are voracious and swallow it completely. It’s fun to try to lip hook them instead, but you have to be on top of it and pull lightly when they jiggle the rod tip to do it.
Carolina rigs work like
@sisQscott says, but they aren’t necessary. A simple mooching rig with a 7/8 oz or 1 oz banana sinker an 18”-30” leader, and a #2 to 1/0 hook is enough. Sometimes I’ll go to 1.5 oz sinkers if the current is running a lot. I usually use 8-12 pound mainline and a 15 pound leader. At Hat Island or Camano or south end of Whidbey or even along the coastline by Mukilteo, I’d usually pull in 10-15 fish an hour with that rig. I’ve used it in the islands too, but not nearly as much.
Flounder, like Halibut, will bite on a rubber glove if you give it to them. They’re not usually that picky with bait. I used to fish with cut bait but switched to plastic years ago. Dogfish love cut bait, but rarely take plastic so it’s a great way to avoid pulling a lot of them up. I’ve had times when I couldn’t get cut bait to the bottom because the dog fish took it on the way down. Plastic fixes that. They’re not picky with the plastics either. I’ve used a lot of sand worms, they work great, but I’ve also used cut bits of larger plastic I had sitting around from other fisheries. Something sort of long and skinny like a worm is all you need. Some days they like red, but if they aren’t biting on red that day switch one rod to yellow or white plastic and that usually changes things up.
Good luck! Flounder are great fun for kids to catch. There are so many of them and since they dive on the bait and swallow it, they’re hard not to catch. Just make sure you bring a hook remover or you’ll be doing surgery to get your hook out.
