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.

This is an interesting project. You have identified 2 critical variables in photosynthetic rate. Two others would be CO₂ availability and plant species. This second variable is what complicates your question. Some plants can use a whole lot of light effectively. Other plants have evolved in shady conditions and can use only a relatively small amount of light before photosynthetic rate levels off.

I suggest that you measure this yourself. Try using an aquatic plant like elodea (any aquarium store will have it). Expose it to various amounts of light. You can measure it with a light meter or just use distance from the light source as your variable. You can measure photosynthetic rate by counting oxygen bubbles because the bubbles will all be about the same size, or you can capture the oxygen and measure the volume.

Adding different amounts of bicarbonate (baking soda) will allow you to see how adding CO₂ will influence rate of photosynthesis. First boil and cool distilled water to drive off dissolved CO₂ so that you can control this.

Obviously, I have not answered your question for dealing with the amount of water. This actually is a more difficult thing to test because measuring the oxygen produced is harder when you are not using an aquatic plant.