A group of penguins swimming together is commonly called a raft.
The term describes penguins traveling or resting together on the surface of the sea. Penguins are highly social birds, and grouping in the water can help them locate feeding areas and respond to predators. The word “raft” is also used for several other water-associated animal groups, so it is not exclusive to penguins.
On land, penguin groups are more often described with terms such as colony, rookery, or waddle, depending on the context and source. In the water, however, “raft” is the conventional collective noun readers are most likely to encounter.
Penguins spend much of their lives at sea, where they hunt fish, squid, and krill. Their streamlined bodies and flipper-like wings make them powerful swimmers, even though they cannot fly through the air.