The free movement was in there from the beginning.
This is a shot from the first Gamemaker prototype mockup.

(It's not very pretty sorry)
WASD to move and mouse to aim and shoot.
The watergun was the primary weapon. And it worked quite well with full free movements. You can sort of spray battle enemy hosers and firehydrants. And do some relatively funky tactical moves.
But as told.. completely free movement + free aim didn't translate very well to mobile in a runner. That would have been a very different twin stick shooter game
And I HAD to scope something simpler then fairly large 3d complex environments. You would associate with a twin stick shooter. All depending naturally.
I still think there is some interesting gameplay with a primary watergun hosing setup somewhere to be found. Perhaps in conjunction with a physics based environment. That could be lots of fun. Something local/online multiplayer water battle something. But that's a completely different project.
I'm very happy for the design solution that came to be.
So we had to switch to a single axis for movement. That did work quite well. Only problem.. having a watergun didn't make a whole lot of sense suddenly. It's was ok'ish. But not really super fun. And I want super fun above all else.
So we went all in on doing waterballons as your primary weapon. Which are quite excellent! The whole precision aiming.. jumping and shooting part is just fun! In my opinion anyways.

There is just something fundamentally satisfying in a soak "kill". That is the result of your player skill based on a sense of timing and distance.