The Oil Drum is an extremely helpful item to acquire since it holds thirty full servings of water. Additionally, it's simple to get. Wait for a tsunami to strike, and then head to Beach 3 the following day (it's the beach nearest the jungle). Pick up everything on the beach (such as seaweed, rocks, oysters, et cetera). You won't be able to hold everything there in your inventory, so continue to discard these items as you pick them up until there's nothing left on the beach. Then, head back into the Water Cave, go back to Beach 3 thereafter, and the Oil Drum will be waiting for you to grab on the beach.