2. Islands are surrounded by water on all sides. (surrounded уже подразумевает "со всех сторон")
3. UK is made up of four countries.
4. A lot of Belarussian cities and towns are linked by railways.
5. Continents are separated from each other by oceans.
6. The biggest part of the Earth is covered by oceans. (А еще лучше most of the Earth. Привет составителю задания)