when you throw a technique in water, as you say, there is resistance greater than experienced in the air around you, resistance is a tool for improving strength, so why do I dispute the logic of power through water resistance? you ask.
i think that we all agree, no matter what style we study, that bar from a small number of specific techniques, our power is generated from our legs, hips and core, then merely transmitted into relevant limbs as tools of contact.
By removing gravitational force from your techniques, you prevent the 'anchoring' necessary for correct power generation, and the effort moves into the muscles of the limbs as they force themselves through the water. I am not against having stronger arms, adductors and abductors, but they are not what should be concentrated on if a hard strike is your goal.
By using the supporting buoyancy of water, you instantly alleviate stress on the core muscles- the very things that need to be worked and strengthened, not nursed and given the day off when you train.
On the question of balance, due to the support of the water, your reaction time needs to be much slower to rectify your balance in water than on land, thus you are conditioning yourself to react slower, not better.
Their is also a subtle difference in the biomechanics of a strike in water, as you are not using your energy purely to drive a limb forward under force, but also using effort to prevent your body moving away from the punch. Familiarising your body to expect such an alien reaction when on land hampers your muscular coordination. I know that you need to bare down on a strike on impact to transfer the full energy into your target, but to condition yourself to experience this resistence through the whole range of the movement is counter intuitive to the speed and accelaration needed for a hard punch. You are effectively 'pushing' your strike, and as we all know, a 'pushed' punch is as much good as a jelly full of pudding.
As with any physical persuit, the conditioning training and the activity are not the same things.
Learning to use your muscles explosively, with excellent balance and neuromuscular coordination within the environment you practice your art, will do you far better than reverse punching your local pool into a froth.
movements like cleans, jerks and snatches, along with concentrated short burst power drills on a heavy bag condition the body to be fast and powerfull, it is then up to you to apply this increased power to your techniques.
A bullet fired through the air will kill a man, a bullet fired into water breaks up, its power dissipated after 1.5 meters. Why condition you body to have its power thwarted?