Genaration bug

When I create a superflat ,,water world" with ocean biome none structure want to geneate no monuments no shipwrecks no ocean ruins even seagrass and seaweed.Only thing that spawn is fish.Don't worry i had generate structures on and still nothing.It look bad because on other types of flat worlds structures etc.villages,grass and cactus are gennerating normaly.Please fix that.