Water Gun vs Waterfall vs Dragon Breath

I’m assuming you mean to power up for PVP because why else put resources into one? If so, both water gun and dragon breath are very good moves in PVP, but dragon breath is awesome. It’s what I’m using.

Dragon Breath / Outrage works well for me on PvP.

Waterfall/Hydro Pump has the odd use in clearing gyms but generally there are other better options.

For PVP:

Water Gun is 3 damage and 3 energy in 1 time tic. (3 DPT and 3 EPT)
Waterfall is 10 damage and 8 energy in 3 time tics. (3.33 DPT and 2.66 EPT)
Dragon Breath is 4 damage and 3 energy in 1 time tic. (4 DPT and 3 EPT)

Waterfall deals more damage by itself than Water Gun, but given how important charge moves are, Water Gun's higher EPT is probably more important.

Dragon Breath's base numbers are strictly better than Water Gun, but dragon type is only super effective against other dragons. With a 1.6x damage for type advantage, Water Gun is boosted to 4.8 damage.

Dragon Breath if for PvP, it's one of the best fast moves and probably a large reason why Kingdra is ranked so high in the first place.

My powered up Kingdra have DB.