Frankly the only way to get to Sodwana is with a rental car if you travel alone. Remember diving at sites like Sodwana are surf launches, and the authorities there restrict vehicle access to the beach where all the boat launches take place, so you will need to book accommodation or dives with an approved operator with 4 x 4 for access to the beach, otherwise you will have to park your car in the park lot and carry everything to the site yourself, all the Sodwana operators below will transport you and all your gear from their accommodation sites to the launch site and back with 4 x 4 vehicles.
To dive along the Natal coast in SA now you also need a local dive permit as the areas are proclaimed marine sanctuaries, and the funds generated from the sale of the permits are used to protect the local marine areas, they are cheap roughly $5-10 dollars depending on the duration, but you must have one, diving without one will incur a hefty fine if you are caught, you can buy them at all the local post offices here, generally all the good dive operator's will sell them as well and none will take you out without one.
Some dive shops run tours to Sodwana Bay and have a minni bus which generally leaves from Johannesburg.
Driving from Johannesburg to Sodwana is a 7 hour trip and the roads are not all that great, driving from Durban is also a roughly 4/5 hour trip, its very doable from both areas, but if you can, my suggestion would be to try and use a dive shop travel operation.
That said Sodwana is an awesome dive site, probably one of the best in SA, good dives abound and the reefs are fairly shallow at around average 15 meters. Well worth the effort.
Aliwal shoal is an easy access dive site and there are a number of local places for accommodation who will take you from their place to the launch site or of course you can just rent a car and drive there if you want to stay off site. Aliwal is great to see bigger pelagic's, there are two good wreck dives and of course always the possibility of a shark sighting - the diving is a little deeper with the average been 18-25 meters and the visibility, although it can be stunning, is usually not as good as Sodwana.
Some suggestions -
For Aliwal check out - Aghulas house,
Agulhas House - Umkomaas Accomodation | Aliwal Shoal | Scuba Diving | PADI Dive Courses or Aliwal dive centre
Aliwal Dive Centre, scuba diving charter, dive school and PADI 5 star Instructor Development Centre or Umkomaas Lodge
Lodge Accommodation | Scuba Diving Resort | Aliwal Shaol Diving - all excellent operators who will look after you well.
For Sodwana trips a good operator leaving from Johannesburg is Ocean Explorers -
Ocean Explorers - good operators at Sodwana Bay are Coral divers
Coral Divers - Dive Centre situated in Sodwana Bay, South Africa. or Sodwana Bay Lodge -
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Feb. in Sodwana, and Aliwal actually, is excellent for diving and the weather is hot - dont forget the sun screen.!
Have fun.