Slack water is between 3 and 4 hours after high water Portland. From there it depends which boat you are meeting. Dive centres are in the old dockyard area. Slips are available at Weymouth, Portland, West Bay and Lyme Regis. Harbour and launch fees are payable.
Campsites are usually smart and a bit expensive.
Technical diving is not required, but it is an ideal depth for extending bottom time with a nitrox mix. Admiralty Chart 3315, Berry Head to Bill of Portland. Ordnance Survey Map 194, Dorchester, Weymouth and surrounding area. Dive Dorset, by John and Vicki Hinchcliffe. Shipwreck Index of the British Isles, Volume 1, by Richard and Bridget Larn.