I found a great spot that is set apart from the battleground area even though you get flagged as soon as you enter. You're in Wintergrasp, there's not much you can do about that, but I didn't see one single person while I was there. Wranngar can hold his own but it has been a looong time since I did any PvP on Gomorra.
I took the flight path to Fordragon Hold in Dragonblight. From there I flew southwest to the southeast corner of Wintergrasp. I flew over the Burning Cauldron and got dismounted for being in a no-fly zone but you get a parachute so it's not too bad. You want to try to land on the big piece of ice floating in the middle of the lake you are heading to. I missed on both Wranngar and Gomorra and ended up in the water but you should be able to do it :).
Once I got there I picked a spot right at the southeast end of the lake (77,70) and started fishing. From what I read you want to use a +100 lure even at maximum fishing. I didn't have any problem on Gom with his 422 fishing skill, +30 pole, +3 fishing line and +100 lure. That gave him a total of 555 fishing skill. It took me about 5 minutes on each to catch my 10 Terrorfish. Once I had my 10 Terrorfish I hearthed back to Dalaran and turned them in to Marcia Chase. The reward for this quest is a Bag of Fishing Treasures which can contain anything from pets to epic items to junk and anything in between.
So I had good luck in the southeast corner of Wintergrasp and got in and out on both without any PvP resistance. I did this during a time when we were in control of Wintergrasp, but where I was fishing I don't think it would have mattered if the Horde controlled it or even if we were in battle.
Update: I have had just as good luck flying into the Alliance flight point in Wintergrasp and fishing in the Sunken Ring right there. If you have the flight point it will be quicker and easier to just fly in, fish there, and fly out.
This was the last of the 5 daily fishing quests Marcia Chase had for me to do so when I turned these in on Gom and Wrann they both got the achievement Chasing Marcia.
Good Luck :)
-Wranngar