>>30397205Seadras are already good for attacking.
Water Gun / Hydro Pump, or Dragon Breath / Blizzard.
Anyway, Kingdra CP range at lv 40 will be: (2066 CP / 118 HP) to (2424 CP / 130 HP). It'll be a better attacker than a Seadra but maybe you shouldn't hold your breath on whether it'll be a good gym filler.