Hi and welcome. Most of my best coastal foraging in Scotland (actually in the UK) has been on the west coast, in particular on the islands and the Ardnamurchan peninsula. The waters there are crystal clear and fish and shellfish have been plentiful at times.
I would download a copy of 'Eat the Beach' by Fraser Christian (he can occasionally be found on the forum) and/or get a copy of the River Cottage Edible Seashore book. The latter doesn't cover fishing but does include molluscs, crustaceans, plants and seaweeds. Frazer includes fishing.
I probably wouldn't favour the Firth of Forth for coastal foraging because of the influence places like Edinburgh could have on the water quality. That may be unfair but I'd certainly check the official water quality statements. The East coast north of Dundee has some excellent places. I've spent a little time up there but not enough.