As other users already stated, Splash does not deal damage.
I'm not sure, but I think I somewhere read something about a bug where, when you re-arrange the attacks of a Pokemon, the game doesn't get it and re-orders only the attacks shown and the messages, but not the effects. So, you could for example get a Protect that is called Relic Song both on your screen and in its message, but which will still have the Protect animation and say "insert_Pokemon_name_here protects itself!" (Not sure whether there are Pokemon with both in the moveset, but never mind.) (Whether or not there are Pokemon with both in the moveset is irrelevant, because of what I explained further below.)
So that's what might have happened to you, you used a Tackle the game called Splash.
Another idea: Did you play an original game, or a patched one? Because if you're using patches, the hacker might of course have changed the code for Splash so it occasionally will deal damage.
If it does come from a hack, you should remove that hack, because a hack that actually wastes space for modifying Splash is way too low quality for being worth it. Only the truly brilliant hacks are hacks you should load, and even for them you should think at least twice, because of the risks. I'm both talking about load errors breaking your game for good, and that the game developers could go after you for modifying their game - a game is copyrighted material, after all.
Also, what "swapped" attacks might truly come from (by now I'm pretty sure that I dreamed the bug part), is an attack randomizer. There are actually attack randomizers that swap the attacks, but then swap back the attack name properties so the attacks appear like the original attacks. If your Splash is no longer Normal-typed, it's almost ceratainly a randomizer because no hack would hardcode Splash to a different type, except perhaps Water, because that just makes no sense.
Randomizers might go as far as swapping back the description, but that's unlikely too, just like hardcoded description changes. So, if your Splash has a different description, it's also randomizer confirmed.
Personally, while randomizers do add difficulty, I don't see much of a point in them BTW. It's a kind of silly difficulty increment IMHO, like enemy spamming in other games.