(covers information from several alternate timelines)
Samuel Clemens' pocket watch is found under San Francisco
A pocket watch was a small Terran timepiece usually worn with a chain fob and carried in the vest pocket.
In 2267, Dr. McCoy witnessed the White Rabbit using a pocket watch on the Shore Leave Planet. (TOS: "Shore Leave")
The Excalbian image of Abraham Lincoln consulted the image of a pocket watch in preparation for beaming aboard the USS Enterprise. (TOS: "The Savage Curtain")
In 1893, Samuel Clemens left his watch in an abandoned mine under 19th century San Francisco, where it was recovered 475 years later along with Data's severed head. (TNG: "Time's Arrow", "Time's Arrow, Part II")
The Persistence of Memory, a painting by Salvador Dali, depicted melting pocket watches. The painting was in the possession of Kivas Fajo in 2366. (TNG: "The Most Toys")
Tolian Soran carried such a timepiece with him while attempting to carry out a plan to redirect the Nexus energy ribbon. (Star Trek Generations)
The replica of captain Cray's watch
In an alternate 2374, Commander Chakotay tried to give Captain Kathryn Janeway a replicated replica of a pocket watch worn by Captain Cray, a sailing ship captain who got his lost crew back home centuries earlier for her birthday. At the time, the ship was under constant attack by the Krenim and she refused it, saying it was a luxury and could've meant a meal or medication for another crewmember. She told him to recycle it, but, unknown to her, he could not bring himself to do so. She found it in his quarters five months later and put it on. Neelix thought it looked handsome on her. (VOY: "Year of Hell", "Year of Hell, Part II")