Shiosai no Himei

The protagonist finds himself as a lowly janitor on a luxury liner when the company he works for after graduating from college goes bankrupt and a friend's betrayal leaves him with 20 mil yen debt. When he hears that a class of girls from a rich private school charters the liner he schemes to lock some of them up, have sex with them and extort money from them to pay his debts.

[From a post on Hongfire forums by davidlu9837]