When unrelated organisms develop a similar biological trait, this is considered convergent evolution. In fact, although penguins appear to swim underwater, they are technically using their wings to fly underwater. They have a layer of blubber under their skin for insulation. Their body is streamlined, an adaptation for swimming easily as their body can cut through the water, propelled by their wings and their webbed feet are used for steering. Dolphins also have blubber to help insulate their bodies. They have a streamlined torpedo shape allowing for faster movement in water. Because many of the same characteristics are in both the penguin and dolphin but were arrived at via different evolutionary pathways, one would call this convergent evolution.
Dolphins and penguins evolved to become more similar to one another over time. Our modern day dolphins evolved from mammals that walked on land, whereas, penguins evolved from birds that flew in the air. Both organisms developed similar structures/features (i.e. flippers because they both swim in the water). This is an example of convergent evolution. Convergent evolution is when two or more dissimilar species evolve to become similar to one another.