Welcome to Science Buddies! This sounds like a great project and you have some really good questions.
When doing a science project, the best place to find answers is to look in the scientific literature and then find a method that is suitable for your research objective. It sounds like you have already been reading some literature, but here is some additional information that I found:
Here is an article where the authors used 5 x 40 cm pieces of plastic bags weighing 300 mg and incubated them while shaking at 24 degrees Centigrade. The pieces of plastic were weighed every 7 days to the closest milligram for 49 days and the pieces were dried with a fan before weighing. This looks like a good procedure if you have access to a scientific balance with an accuracy of 1 milligram.http://plasticsoupfoundation.org/wp-con ... urtles.pdf
Here is a comprehensive review article that describes several methods for testing degradation of plastic bags. This article has lots of references in the bibliography that you might want to look up. The background information would be useful for your research paper as well. http://igem.molgenrug.nl/iGEM2010/brain ... dation.pdf
There is a standard test method for measuring the degradation of plastic bags that has been published by the American Society for Testing and Materials (ASTM). You should try to contact this organization and ask if you could obtain a copy of the test method without having to pay for it. The science fair judges would be impressed if you used the standard ASTM method, but you can use any method as long as you describe exactly what you did. http://www.astm.org/Standards/D7475.htm
Your idea for an experimental design sounds like a good one to me. Since you want to compare seawater and fresh water, you would want to maintain all conditions identical, except the source of the water. The controlled conditions for the experiment would include the temperature, UV light exposure, movement of sample. Your experiment would yield data that would be meaningful if your experiment is completely controlled and if you include at least duplicate samples and very accurate and sensitive
This experiment is a little complicated, so it would be good to set up a pilot experiment as quickly as possible to verify feasibility and then set up a definitive experiment. Your experiment time should be at least 49 days, but if the ambient temperature is less than 22 degrees Centigrade, you would probably have to extend the time. Since you are in San Diego, I don’t think temperature will be a problem, but you should probably plan to measure the temperature during the experiment. It will get a little cooler, so an experiment done in October/November will go slower than an August/September on.
The type of scale that you need with a precision of 1 mg is very expensive and it would probably be good if you had access to a research laboratory with this equipment. You might look for pricing of a used scale from a local company that resells scientific equipment. I don’t know of anyone in San Diego, but you should call any local colleges and private laboratories close to you and explain what you need. I think that measuring the results over time (every 7 days or so), would yield excellent data for analysis, so you need access to a balance that you can use for an hour or two every week.
The first paper I cited above used fans to dry the plastic strips before drying and this seems like an excellent method. It seems like using an oven could easily expose the plastic to temperatures that would melt the plastic (uncontrolled parameter). While you are looking for used scientific equipment, also ask about a vacuum oven. It seems like using a small vacuum oven at a low temperature (30 degrees Centigrade) for an hour or two would be a really good method for this step. You should probably do a pilot experiment and determine a standard method for you to use for this important step. To validate your method, you would weigh the dried strips after a period of drying and then continue the drying and weighing until the weight is constant (no water left to evaporate).
Plastic buckets would probably be better than metal ones. The metal containers would leach metal ions that could affect the growth of the microorganisms that are degrading the plastic bags. Glass would be the best material as it is inert, but it is breakable and much more expensive. Whatever container you use, make sure you use identical containers so that this is another controlled parameter in your experiment. You don’t need really large containers if you will be starting with plastic strips that weight 1-2 grams.
You have time to do 2 x 49 day experiments before your project is due, so I would recommend starting as quickly as possible so you will have time to analyze your results. You will learn a lot from the first trial and probably improve the procedure, so your first test should be more of a small test run before the larger definitive experiment. If you run results in triplicate for the second experiment, you will have good data for statistical analysis.
Instead of using an artificial UV light, can you should consider using natural sunlight to simulate actual environmental conditions? There is plenty of great sunshine in San Diego that would work perfectly if you can work outdoors. If you did use UV lights, you would need to set up the experiment in a closed room, and turn off the UV lights before entering the room. Otherwise, the conditions would be unsafe.
I think I have answered all of your questions, but let me know if you think of anything else.