Is there any mention in the Prophetic Sunnah that indicates the jinn inhabit the sea, and a prohibition against swimming in it at night for fear of harm?

The narration attributed to Bilal ibn al-Harith, which specifies the dwelling places of Muslim and polytheist jinn (villages and mountains for Muslims, and between mountains and seas for polytheists), is considered very weak by consensus.

However, the Quran indicated the ability of devils to dive (وَالشَّيَاطِينَ كُلَّ بَنَّاءٍ وَغَوَّاصٍ - "and the devils, every builder and diver"), which suggests that seas could be a refuge for them, as they are not inhabited by humans. They also inhabit ruins, solitary places, and places of impurity.

What strengthens the possibility of jinn dwelling in the seas is that the throne of Iblis is upon water, as narrated by Muslim.

It is desirable to be wary of places believed to be dwellings of jinn, and when there is a need to swim in them, it is recommended to fortify oneself by remembering Allah, seeking refuge [in Him], and reciting Ayatul-Kursi.