Slack is 1 hour before high water. Contact the Long Room beforehand to check no shipping movements and OK to dive You jump off a small jetty which is to the north west of the car park. From here swim north (inland) following the contour of the shore. You can get down to 40m where there are standing stones. Come up and you get to the underwater stone wall of the Royal William Yard, around 5-10m, follow this south till you get back to the entry point, there are stone steps and a handrail to get out of the water. If you time it right the outgoing tide drifts you back. Lots of lobsters, conger, dead men's fingers etc.
Use multimap.com, search for Admiralty Road, Plymouth PL1. car Park at the end of Admiralty Road, from there a short walk to the drop off.