Another option is to take it on the Torpoint ferry (£3 including the car only charged one way) into Plymouth, from there you have a good slipway within 5 mins heading towards city centre. It's called Richmond walk. Launches right at the mouth of the tamar, which means your just seconds from the Plymouth Sound - breakwater and beyond or can head back down river towards Torpoint (5 mins by water) or Saltash (15 mins due to speed limit)
Here's a link
If using a sat nav the post code for the marina is
Devon PL1 4LS
Just go past the Marina and you'll get to the slip. Free parking (limited). A little more "remote" or tucked away so take a lock, or three.
Peter @ Boatsandoutboards4sale ~ email@example.com
~ 07930 421007