Splash makes a great scouting move on all choice variants of Magikarp.
When using it, you run no risk of your opponent switching in something with volt absorb, motor drive, flash fire, and water absorb and leaching 25% HP, a fire attack boost, even a speed stat boost! Its a very safe and effective move to see what your opponent is going to do.
Now, some people ask "why not use something like rain dance or sunny day. Both of those wont activate those abilities, so why not empower your team through the use of well placed weather?
The retort, of course, is obvious. You use an WHOLE turn setting up a weather effect, and then a Tyranitar comes in and changes it to sand, WITHOUT using a WHOLE turn. Weather is useless, so don't try and act like it isn't. So Magikarp wins again because it wont have you wasting turns.
Further, Magikarp has some nice stalling abilities in Bounce once you have scouted your opponents team to really rack up that Toxic Spike and Toxic damage. Flail is great for when your Hit Points are low from switching into Stealth Rocks and Spikes so much, since most Karp will be in and out all game. Flail is a unique [almost] move in the fact that as the battle progresses, it gets stronger. When Magikarp's Hit Points fall below 4% of their maximum, its TWO HUNDRED power. That is literally twenty stronger than Garchomp's Outrage attack.
So as you can see, splash makes a good move to round out Magikarp's moveset, combining the scout, sweeper, and staller into one pokemon is one amazing feat.
See, now I have to say: I told you so. ~Mirage312 King of PBR.