I mean if you have the event Magikarp with Hydro Pump or a hacked Magikarp with any water-type moves like Surf. I think so since it would make sense but without event or PowerSaves it's impossible to get a legit water-type move on Magikarp. Would it get STAB? Just wondering
Here is the event Magikarp that i'm talking about: http://www.serebii.net/pokedex-xy/129.shtml
Yes there is no reason it why it shouldn't work, even if it is hacked; the move gets STAB if it's type is the same as the user's.
Source: Knowledge
Yes, it does. There is an event magikarp with hydro pump to ensure it too.
In Sword and Shield you can Teach Magikarp Hydro pump with TR03, in Sword and Shield is just that easy, the only hard part is getting a TR03 (To me atleast)