Photosynthesis is light dependent. At low light intensities, light is a limiting factor for photosynthesis. The rate of photosynthesis to the light intensity is inversely proportional, so it will increase rapidly at first the levels off until it reaches its optimum speed. It cannot photosynthesize any faster after that because there is only a certain number of chloroplasts containing a limiting amount of chlorophyll and this can only absorb a certain amount of light which is called the light saturation point. The only relationship I found was 1/d2 (distance square).
Since there are many steps involved in photosynthesis and each step has its limiting factor, it can not be this simple.
The net reaction of photosynthesis is
6H2O + 6CO2 ----------> C6H12O6+ 6O2 . You can see that you definitely need water for the photosynthesis to go on. But we say that water availability is only indirectly limiting photosynthesis because for the C4 plants, carbon dioxide is often the limiting factor.
You can check these papers for references:
1. Kitaya Y, Okayama T, Murakami K, Takeuchi T. Effects of CO2 concentration and light intensity on photosynthesis of a rootless submerged plant, Ceratophyllum demersum L., used for aquatic food production in bioregenerative life support systems. You can find it here.
Adv Space Res. 2003;31(7):1743-9.