I need to do some serious cleaning of the sea chest on my boat. The sea chest has all of the thru holes supplying raw water to the systems which use raw water. The internal size of the sea chest it's 12" square by approximately 36" high on the onboard side of the chest. On the bottom of the chest is a bronze grating. I am planning on removing the top of the sea chest so that I can reach in and do the cleaning. However and here's the problem,  the top of the sea chest is just below the water level outside the boat. I have a diver coming in a few days and I plan on using a10 mil tarp to put over the bottom of the chest, over the grating thereby stopping most of the water coming into the sea chest. I will have extra pieces of tarp available just in case. What thinks the list, will the tarp be sufficient, any suggestions what to use. Thanks Paul Koch