Common sense method I used last week to win the Stranglethorn Fishing Extravaganza.
All you need to do is download the addon ServerHop (x10 better than Cross Realm Assist).
Say you find a good spot to fish during the contest, but as always 50 people suddenly decide to show up there, just hit "next group" and it will port you to a new realm with new pool spawns.
No need to waste time flying to a new spot with less people, just realmhop for a few seconds and you're good.
A good spot I found for pools was the Wild Cove just south of Booty Bay, but it may be a bit busy for some people. Loads of pool spawns, though.
You need to use ServerHop for this and not CRA as SH has almost 0 lag on joining groups. CRA is slow as hell.
Note: You cannot win by hopping to a realm which has no winner yet to hand in the quest. When you hand in, it automatically does it on your own server.
TL;DR: If you get competition at your favorite spot during the STV fishing contest, just realmhop.