It will be tight to get in Rum Point (and spend a respectable amount of time there), then make it back to the ship before 4:30 for the last tender. Not saying it cannot be done, but you will be hurried to do it.
What time does your Captain Marvin's tour get you back to the marina? From there, if you left immediately by car it would take 30 to 45 minutes minimum to get to Rum Point. Then another 30 - 45 minutes from Rum Point back to the tender pier.
Others who experienced this could give more reliable estimates, but it is a drive to Rum Point.
Good luck!