Is long sea travel (cruising) forbidden because of the hadith: "None should embark on the sea except for Hajj, Umrah, or fighting in the way of Allah, for beneath the sea is fire and beneath the fire is sea"?

The aforementioned hadith is weak, by the consensus of the Imams. Imam Al-Bukhari, Abu Dawud, Al-Khattabi, and Ibn Abd Al-Barr all explicitly stated its weakness due to the anonymity of its narrators. As for sailing the sea, it is permissible according to the general opinion of scholars, due to the weakness of the aforementioned hadith. They permit it for seeking lawful provisions, as long as it is not for excessive worldly gain and wealth accumulation.