This solution is only applicable to those who can host an achievement world from a PC, tablet or phone!I've made an achievement world which has both phase 1 and 2 achievements in there pretty much set up. Download it from here:
https://www.dropbox.com/sh/90bviwj4dhz8i83/AAATWKpkwXEVNQ8ai...Aquatic Sh0wt1m3 is the file for MCPE versions (which is in version 1.5)
.W10_Aquatic Sh0wt1m3 v1 for 1.5.0 (Thanks to Mathguy42) is for WP and Win10.
For the achievements, open chest on left. Retrieve the Prismarine blocks, Trident, raw salmon and sea pickles from the top row.
Jump in the water and go underwater. Have the trident in your hand and hold LT/right mouse and then release to give your self a boost for
"Do a Barrel Roll!". Go to the portal near the shore and place the missing prismarine block on the center bottom (directly beneath the conduit) to unlock
Moskstraumen. Now go to any random sand block on the floor in the ocean and place 4 sea pickles on the same block to unlock
"One two sea four pickles". Last thing to do is to feed salmon to a dolphin. Just swim around for a bit and once you spot one head over to him and feed him the raw salmon to unlock
"Echolocation".Note: If unsure how to extract world, have a look at the walkthrough's hints and tips page.