This is the 11948th most frequent Russian word.
затонувшему
"To the sunken" or "to the drowned."
The word 'затонувшему' is used here as an adjectival participle describing the ship that has sunk.
Я подошел ближе к затонувшему кораблю.
I approached closer to the sunken ship.
In this sentence, 'затонувшему' serves as an adjective modifying 'катеру', indicating that the boat has sunk.
Спасатели пришли на помощь затонувшему катеру.
Rescuers came to help the sunken boat.
Here, 'затонувшему' is utilized as an adjective describing 'парому', again highlighting that the ferry is underwater.