At first glance, it might seem that a seal is more closely related to a fish than to a deer, for a seal spends much of its time in the water. Yet when we look more closely at the characteristics of a seal, we discover that it is actually more closely related to a deer because both a deer and a seal are mammals.
To determine this, we need to examine the characteristics of mammals and of fish. Mammals have fur, but fish certainly do not. Mammals give birth to live young, but fish hatch from eggs. Mammals have warm blood, but fish have cold blood. Mammals breathe with lungs, but fish get oxygen out of the water with gills. Sea mammals use their front flippers a bit like arms or hands, and they also use their front flippers to help them swim, but fish do not do so.
Now let's examine the seal with regard to these characteristics. Seals definitely have fur; in fact, baby seals have fluffy white fur that makes them look like little puppies or teddy bears. Baby seals are born live, and all seals are warm blooded. Seals also take in air through their lungs, and they can and often do spend time on land. They do not need to remain in the water to stay alive the way fish do. Seals can also use their front flippers with great dexterity. Just think about the seal at the zoo that claps its flippers and does other tricks that a fish would never be able to do.
We can tell, then, that seals are definitely mammals, and therefore, they are closer relatives to deer than to fish in spite of appearances.