i have heard somewhere a long time ago, which was basically making a horseshoe shaped dam of rocks with a hole behind it
Up here its called a Flounder pond, its probably called the same thing everywhere, when the tide goes out you go in in your bare feet and try to find your flounders by treading on them.
Wilks, (Winkles in England) are easy, as are limpets, both are grazers as opposed to filter feeders and as such don't concentrate any toxins that might be present.
Crabs are very easy to catch with a drop net, and if your lucky and over a deep pool (off of the edge of a cliff) you might get a lobster.
Sea weeds are fairly easy to collect, but personally are not to my taste, but luckily an amazing variety of edible plants seem to grow on the shore. Caloric staples to be foraged are Sea Kale roots and Silver weed roots ( a famine food in Scotland) as well as more common staples. Plus a host of greens like Sea beet, Black mustard, Sea kale stems (blanched) Wild radish. All of these grow prolifically on the shore where I live. Plus all the usual fare you find inland.
I think its wise if you think you may be stuck in this sort of situation to carry a fishing rod and reel, they are relatively light and could fill your belly day after day.
Sea birds are another thing to consider, Gannets have been staple fair in the highlands in the past. Birds can provide you with fat where game has very little.
Just as an after thought, the River Cottage handbook on the seashore is really excellent i think, has a good explanation of legal stuff and lots of good info.
My other favourite book is The complete book of seafood fishing by Rob Avery, its not a kit monsters book, its just what you need to know to get started with a lot of different stuff, with an emphasis on food.
