Question
Hi, I am a middle school student named hunter who is researching the effect of saltwater on grass growth for a science fair project.  I have a few questions about the growth of grass. In the range of 7 to 8 weeks can the grass growth to the fullest?  Does grass grow faster without water or with water?  If you cant answer these questions can you refer me to  anyone or recommend any options. Thank you for giving me your time.

Answer
I'm glad you've taken an interest in biology at your age.  To start, there are many different grasses with varying maximum heights and nutritional needs.  There are even grass flowers, and a few species of sea grass(e.g. Posidonia oceanica), which can only live in saltwater.  Grass is a plant like any other in the plantae kingdom, but what we consider to be "grasses" are those plants found in the gramineae family.  Grass, like other plants, is comprised of roots, stems (stolons ans rhizomes), leaves, and sometimes flowers.

Grass grows by way of photosynthesis.  Photosynthesis, simply, is the process that allows a plant to obtain energy from sunlight.  The chlorophyll uses the light energy along with water(H2O) to convert carbon dioxide(CO2) into carbohydrates, with a byproduct of oxygen(O).

SUN + CO2 + H2O + nutrients --------> ENERGY + O
                          chlorophyll

It's a little more complex than this equation, but that's about the jist of it.
Therefor, water is an essential to the growth of grass.
It wouldn't be surprising to see grass grow to its maximum height in 2 months, it would just need optimum conditions, which are hard to achieve.  There is a whole science behind plants and soil conditions.

The salt will absorb moisture from the soil, dehydrating the plant cells.  It will surely kill plants in a high enough concentration.

Good luck and don't forget to maintain a very good control sample, isolate any independent variables, and assign cause to all dependent variables.