For Portland, continue on the A354 past Chesil Beach, turning left to the old dockyard area as the road starts to climb the hill to Portland. There are no convenient transits, so you have to search with a GPS and echo-sounder. Air, nitrox and trimix are available from Old Harbour Dive Centre. Slips are available at Weymouth, Portland, West Bay and Lyme Regis. Harbour and launch fees are payable.
Campsites are out of town, usually very smart and a bit expensive.
Experienced sports divers who are prepared to make decompression stops. Mixed-gas diving is not required, but you won't get much bottom time without planning a deco dive and a rich nitrox mix is always a good idea for deco stops. Admiralty Chart 3315, Berry Head to Bill of Portland. Ordnance Survey Map 194, Dorchester, Weymouth and surrounding area. World War One Channel Wrecks, by Neil Maw.