Island is a term used for any location that is all surrounded by water on all sides. Peninsula, on the other hand, simply refers to an area that has water surrounded by it too, but aside from it is connected to the mainland. You can journey to the island by any other means of transportation, but not by land. Meanwhile, you can also journey to the mainland through the land.
However, it takes a big body of water to surround a land for it to be referred to as an island. If it is surrounded by just a river, it is not an island. But when the water body is not even that big, it can still be referred to as a peninsula. There are basically two types of islands, which are an oceanic island and Continental Island.
The continental island is located on the continental shelf, while the oceanic islands are located beyond the continental shelf. When it comes to the peninsula, it can only be continental. In addition, islands can exist in-group, whereas the peninsula mostly stands alone.