This is the 14758th most frequent Polish word.
pływającego
"Floating" or "swimming" (genitive/accusative singular form or masculine/neuter genitive/accusative singular participle).
In this sentence, 'pływającego' is a form of the verb 'pływać' (to swim), used as an adjective to describe the child. It is in the masculine, singular form and genitive case because it complements the object of 'widziałem'.
Widziałem dziecko pływającego w jeziorze.
I saw a child swimming in the lake.
Here, 'pływającego' describes 'mistrza' (master or champion), denoting the champion in the act or art of swimming. It is in the masculine, singular form and genitive case, as it modifies the noun governed by 'podziwiam'.
Podziwiam styl pływającego mistrza.
I admire the style of the swimming champion.
In this context, 'pływającego' qualifies the bird, depicting it actively swimming. It takes on the masculine, singular, and accusative form, relative to the verb 'obserwowaliśmy' plus the object.