Instead of looking at bare coastlines (which is a nice shape thus far, btw), why don't you figure some rough topography? Mountain chain >there< suddenly will make continuation of isles off its end as partly submerged mountains seem plausible. If flat land stretches >here< then maybe the sea adjacent is also shallow, with low isles of a more blobby shape than rocky-fjiordish-crinkled shapes - sandbanks and barrier isles like off the US gulf coast, or North Carolina-to-Georgia coast. If this is a mid-ocean archipelago, figure whether it's a scrap of continent, with continental shelves around about, or maybe abrupt seamounts where most of the uplifted land "makes it" above the waves; sharp dropoffs and navigation all the way to the coast. If some of what you've got going is volcanic seamounts, or drowned elder volcanic seamounts, in a tropical/semitropic clime, maybe there's swaths of reefs (coral won't grow below a certain temperature or depth). Scale matters a bit - how far out one might find reefs, or barrier isles, might be so many hundred yards to so many miles -- only tens or hundreds of miles out if it's really flat/shallow seabottom. If there's an oceanic trench off to one side where a crustal plate is being shoved under your archipelago, the whole set of islands could be caused by uplifted seafloor and volcanism, not to mention the opportunity for cataclysmic earthquakes, tsunamis, and such. If you have many square miles of somewhat sheltered supershallow water, there's cause for a maze of mangrove swamp/islets.
Have you got any preconceived notions about the nation (s) involved? If you "need" lots of seaborne commerce, indentations for harbors are more "necessary". If it's a bunch of xenophobes who don't want contact or don't even know of other lands, sheer-cliff coasts and impassable reefs could be plausible cause (or convenient excuse). If you want considerable island-to-island shipping, the spacing matters a bit. If you want a midocean transfer point / safe haven, that inner bay & sheltered passage is ideal... if you want one city filling that niche, and another rival city wanting the same prominence, you could have the second one's harbor/ anchorage sheltered a different direction - one is safe in one season but exposed during a different wind pattern, the other one vice versa.
