Adult PC Game: Mii no Tabi + (Mii's Journey +) Ver.1.03 by Team Niimii Archive

Mii is an adventurer on a quest across the world.
Nii is her strangely immortal cat who has lived for centuries.

One day... Mii and Nii were traveling between islands...
Suddenly Nii went berserk from a fever, and sank their boat into the sea.
They floated to the nearest island, thankfully.
But how will they get off it?