Instead of the plastic though, consider renting your utensils for that large party. There are companies that will rent everything you need, and you don't even have to wash it when you are done. They are designed to use less packaging, and normally cost less to purchase than the regular version of the product. Water Conservation Nature does it. The total amount of water on the planet is essentially fixed.
Nature passes it continually through the water cycle.
Evaporation from bodies of surface water, condensation into precipitation, flowing through streams and rivers, back to the bodies of surface water. It will disintegrate over time standing in the water and may affect the seal of the flapper causing a perpetual leak that will waste far more water than the brick could have ever saved. Put the lid on and place that in your toilet tank to reduce water consumed per flush. Virtually free, reuses a plastic container, saves water and won't damage your toilet. You need the rock to make the container sufficiently heavy that it will not move around as the water refills the toilet tank.