I have won this numerous times and am farming exalted with kaluak on a 19 druid atm. The best way is to use the lake at the top of Howling Fjord. Soon as you see that Blacktip, smash your hearth button. Easier with 450+ fishing but other than that just pure luck.
You get 2500 rep per win and 75 rep for turning in a shark after a winner has been chosen. I myself am also trying to get exalted on a 19, but it's a lot of work..and luck.
won the contest on a 70 and caught the winning black tip shark in open water east coast of howling fjord just above horde base there on a high pop server. my 70 had 450 fishing skill and used:
+30 pole from daily fishing reward bag
+5 fishing hat from daily fishing reward bag
+150 heated treated lure (eng's make)
hearthstone set to dalaran
having the fishing rum +10 fishing, gloves with +5 enchant and the pole enchanted with either the +1 or +5 fishing lines might help even more
getting flown around may actually waste more time and depending on you server pop, may cause more exposure to getting ganked. 1-2 ganks, you are pretty much done. if you don't have hi-level 375+ fishing, you will need to fish from schools which a level 5 plus can get fish from