This is the 4579th most frequent German word.
schwimmend
Floating or swimming.
Here, 'schwimmend' is used as an adjective describing the 'Boot', indicating its state of floating or moving on water.
Das schwimmende Boot bewegt sich sanft auf dem Wasser.
The floating boat moves gently on the water.
In this case, 'schwimmend' is used as a present participle describing the action of the subject ('er') in the sentence.
Schwimmend überquerte er den Fluss.
Swimming, he crossed the river.
Here, 'schwimmend' functions again as an adjective describing the 'Auge', referring to its floating nature.