Yesterday, Tommaso (one of the Minecraft PE developers) posted this on Twitter:
No screenshots of MCPE from me because I got stuck on a terrible terrible rebase of Resource Packs... but ask @argo_major about oceans :)— Tommaso Checchi (@_tomcc) July 13, 2016
With Ocean Monuments being the only naturally-generating objects in oceans, it's likely the surprise feature. Ocean Monuments were originally added in Minecraft PC 1.8, with 1.9 altering them to only appear very deep in the ocean.
And if it's not Ocean Monuments - then what do you think it could be? Let us know in the comments below.
And if it's not Ocean Monuments - then what do you think it could be? Let us know in the comments below.