Honestly best way I’d go about it is to exaggarate it. Have the flow go up and down initially, or expand/contract.

Maybe overlay the stream with some dashes moving around to simulate the flow. And have the turret slightly recoil from the initial blast.

the flow will always start out going very arc-y (low power) then become a stream/straight line

maybe only for a couple frames, but still

you can do that along with some fluttering like arc suggested with maybe some burst effects

since its the type of water gun it is, it should get to full power pretty fast