Where they appear in Seven Seas

They generate as part of the world in ocean biomes, so they appear naturally as you travel rather than needing to be summoned. Because generation happens as land and sea are created, ships turn up in ocean you have not visited yet; water you already explored stays as it was.

They are spaced widely enough that finding one still feels like a find. A boat or an elytra crossing will run into them far more often than coastal play will.