The water at the bottom of the falls would be slightly warmer, because the potential energy from the height of the top of the falls would be converted first into kinetic energy of the falling water and then into heat, warming the water slightly as it crashed into the pool at the bottom of the falls. It has less surface per volume than a sandwich. It will take more heat to increase the temperature of given amount of watermelon than to increase the temperature of the same amount of sandwich. Iceland is an island. It is surrounded by ocean.
Warm air comes off the ocean and warms the island.
Because water has a higher heat capacity than land, it won t cool to as low a temperature for the same heat loss in winter as the land at the same latitude does. In the summer the ocean increases in temperature slower than land does so the ocean keeps the island from getting as hot as the mainland does. The 10 kg jug of water would be better. Water has a higher heat capacity than iron, so it will give off more heat for each degree temperature drop. Desert sand has a low heat capacity since it changes temperature easily.