In order to get the Underlight Emerald you need to complete the achievement "Fisherfriends of the Isles". This achievement requires you to get Best Friend "reputation" with 6 different Broken Shore fishermen. Each fisherman takes about 3 hours to get from Neutral into Best Friend, so the grind is not too bad compared to some of the other fishing grinds.
Don't have max fishing yet? Read how to level up fishing easy with the Ancient Vrykul Rings.
Unlocking the Fisherfriends of the Isles achievementEach Broken Shore zone has one fisherman, and only one of these can spawn each day. Every day a new random fisherman will appear, but you won't ever find two at the same time. The fishermen are as follows:
Impus on the Broken Shore.
Sha'leth in Suramar.
Corbyn in Stormheim.
Ilyssia of the Waters in Azsuna.
Keeper Raynea in Val'sharah.
Akule Riverhorn in Highmountain.
All of these are pretty much like Margoss, and in order to get the item you need to turn in to them you only need to be standing really close to them, so there's no need for a special item like with Margoss. You get a buff when you are close enough, so make sure that the Something's Fishy buff is on your buff bar.
If you haven't heard of Margoss yet, check out this guide to learn how to get him to Best Friend and buy a mount from him!
Once you get into the area and have the buff, you are able to fish up unique fish that you can deliver to the fisherman. Each fish gives you 75 rep, but you can increase this with +rep buffs such as the one from Darkmoon Faire. The fisherman also sell some Toys and pets if you're interested in that stuff.
You unlock the achievement once all six fisherman are at Best Friend reputation, and the Underlight Emerald will be mailed to you instantly! Good luck.