This is the 471st most frequent Ukrainian word.
морі
The Ukrainian word "морі" means "sea" (locative case).
'Морі' is used here as the prepositional case of 'море', indicating the location where the action is taking place - 'in the sea'.
Ми купаємося в синьому морі.
We are swimming in the blue sea.
'Моря' is the genitive case of 'море', showing possession - ['shore' of 'the sea'].'
На березі моря ми побачили чайок.
On the shore of the sea, we saw seagulls.
'Морі' is again prepositional and in this sentence indicates the observed location, 'on the sea'.