Shiva for a Drowned Person

If a person drowned in water and cannot be found, should the family sit shiva for him?

If he drowned in water where a person standing in the middle cannot see land in all four directions, they do not sit shiva for him. However, if he drowned in a cave or a place where it is clear he did not exit, they sit shiva from the time they lose hope of finding him.

Shulchan Aruch, Siman 70, Seif 7: "One who drowned in water that has an end, or if it is rumored that robbers killed him, or an animal dragged him, from when do they start counting? From the time they lose hope of searching." The reason that if he drowned in water that has no end they do not mourn for him, as Beit Yosef writes in the name of the early authorities, is due to the concern that it may lead to error and they will say his wife is permitted, while she is forbidden, as explained in Yevamot, page 121. Therefore, they do not mourn for him.