My mom and I will be in Marseille for three days in late September. We have a lot already booked, but would love to take a short boat ride. I'm trying to find a 1-2 hour boat tour, harbor cruise, etc. either before 12pm or after 7:30pm on a Friday or Saturday. Any recommendations?
Go to Chateau D'If. It is a castle fort in the harbour just west of Marseille. It is famous for being the prison featured in the "Count of Monte Cristo" by Dumas. It is a story that has been made into Movies and TV Series. I was there myself. The trip takes about 2 hrs and is under $20.00. I don't know their schedule.
We really enjoyed a boat ride from Marseille around the Calanques, and I think that would fit your description. I don't remember which company we used but you can find more info here: https://www.marseille-tourisme.com/en/discover-marseille/nature/the-calanques-of-marseille/how-to-access-the-calanques/the-calanques-of-marseille-by-boat/