Only works on US it seems, due to EU having only one realm time.
Just tried this this weekend and came up with a great way to win the fishing derbies. First off you'll need two things, your account with the toon you want to win the event and a real id friend. Thats it.
1. Find servers with the appropriate realm times, start with an est server and keep moving through the time zones.
(I used Medivh for EST, Winterhoof for CST, Kel'thuzad for MST and Windrunner for PST)
2. Have your real id account make a character on the realms you pick. and log into the first/next realm the event starts on.
3. Have your realid send an invite to your character you want to win the event with. This way you should be on the real id toon's server. Check your realm time to be sure.
4. Start fishing when the event starts and keep fishing regardless of who wins.
Next steps depend on the event.
5a. This is the Kalu'ak Derby and if you didn't catch anything repeat steps 2-4 until you do catch the blacktip shark. When you do, hop onto the next timezone server where the event hasnt started yet and wait til it does. When it does turn in the fish right off and voila easy peasy ;p
5b. This is the Stranglethorn Fishing event and ya just finished off your 40 fish, hop onto the next timezone server where the event hasnt started yet and wait til it does. When it does turn in the fish right off :P
6. Profit!!
This should work up to PST and if ya get that far without getting the shark your gonna hafta win this one legit.
Also to note, make sure the realm times in Dalaran/Booty Bay match up to what you need along with the surrounding zones you'll fish in. Some realms have different time in dala/bb than in the other zones.