If you discover a valuable sunken treasure, and you can identify living relatives of the treasure's original owners, must you give them what you find or keep it for yourself?
You may keep it for yourself. The Braita states (for example, here) that a lost item which is unobtainable to the owner - such as a sunken treasure - is automatically subject to ye'ush (see below) and is free to take. There is no legal obligation to return it to its former owner.
Ye'ush means the owner gave up hope of ever getting it back and thereby loses his ownership of the item. If the item is lost in such a manner, such as being sunk at sea, even if he says "I don't give up hope of recovery", the Halacha simply considers him to be in denial, "Like one who cries out over his item that was burned." As soon as the item is lost in such a manner, ye'ush takes place and the item may be kept by the finder.