This is the 2438th most frequent Polish word.
bałtyku
"Bałtyku" means "of the Baltic" or "of the Baltic Sea" in English.
Here, 'Bałtyku' is used in the locative case, indicating the location of the sunset.
Widziałem piękny zachód słońca nad Bałtyku.
I saw a beautiful sunset over the Baltic Sea.
In this sentence, 'Bałtyku' is in the genitive case, describing possession or relation to the coast.
Odwiedzimy wybrzeże Bałtyku w te wakacje.
We will visit the Baltic Sea coast this holiday.
Here, 'Bałtyku' is also in the genitive case, describing possession related to the waters.