Many times I've seen "Oh, a bite!" followed by, less than half a second later, "It got away..." So what's the timeframe to reel it in for these species: Corsola, Staryu, Whiscash, Gyarados, Tentacruel?
Reeling a Pokemon in Pokemon Emerald is not much different than other games that involve fishing. The best thing to do is find the right/appropriate rod and press the "A" button rapidly and quickly once it says "Oh, a bite!". It might take a while to find the right beginning point, but your chances gradually become easier once you get the hang of it. Some Pokemon might appear more likely and appear to become caught more easily in an enclosed spaced (or any small area). Trust me, you're not the only person to experience, and you're not the unlucky one. Just continue and you're start to boost confidence and make fishing a lot easier.
Hope I helped! :)