Yes, fins are used to propel and guide the fish through the water. Fish are also cold-blooded, which means that they have a body temperature the same as their environment. Raise your hand if you think you can name one of the characteristics of a fish. I will write them on the board and, when I am finished, we will decide if we have named all of the characteristics. The next group of vertebrates is amphibians.
Amphibians live part of their lives in water and part on land.
Yes, frogs are perhaps the most obvious one. Yes, they are tadpoles. Tadpoles change and become adult amphibians and are able to live on land and breathe air. They also look quite different from the tadpoles of the early phase of the life cycle. Amphibians are also cold-blooded.