Tantei Kibukawa Ryousuke Jikentan 'Kairoukan Satsujin Jiken'

This case is set in "Kairoukan", a strange building in the middle of the sea, which was built by the former chairman of an architectural firm famous for its eccentricities.
The story begins when you are invited to an event to celebrate the inauguration of the building.

Seven invited guests are at the destination. The owner of the "Kairoukan", who never appears. And then...
A tragedy occurs in the mansion, which is closed underwater.

Who is the culprit? What is the motive? And what is the secret hidden in this building? The truth lies in the dark sea...

[From Steam]