Ocean monument seed: -513070979
Ocean monument is a new kind of structure that can be generated in Minecraft world and its hard to find. These structures are generated underwater and you probably won't notice it, even if you swim right over it. If you use a right seed, you will find Ocean monument in Minecraft PE for three minutes or so.
You will spawn on the bigger island, move into the direction of smaller one, hopefully you will be able to notice monument under the water from there.