Hello I heard about 2 or 3 nights cruise from Cape Town to Durban in South Africa, will it be operated in July and August ?
No, unfortunately not.
These cruises are run by MSC, but only operate during the South African summer (the cruising season is between about November and March).
July and August are the South African winter, and the rainy season in the Cape, so the conditions are too cold, wet and rough for comfortable cruising.
Poor weather and large seas are probably a larger consideration. I would suggest rather travel by road, the garden route is a great trip.
I should add that this shouldn't put you off a trip to South Africa in July and August - you just have to pick the right part of the country weatherwise.
Winter in the Cape isn't the best time to visit - although you can have a great time in Cape Town any time of year, and prices tend to be lower to reflect lesser tourist demand. It's prime right whale watching season, and places such as False Bay and Hermanus offer some of the best land based whale watching in the world.
KwaZulu-Natal is very pleasant at that time of year, and the water temperature is warm enough for swimming and snorkelling. In all honesty, although Durban is OK for a day or so in transit, I wouldn't devote any more time to it, and would rather suggest renting a car and heading north along the coast. You can't go wrong basing yourself in sleepy St. Lucia (South Africa), which offers both a pristine beach and the glorious neighbouring iSimangaliso Wetland Park, where you have myriad activities to choose from - my favourites are snorkelling and game viewing from horseback (if you're a rider, the best option is a ride that combines both beach and game park). If you're a diver, some of South Africa's best diving is found close by at Sodwana Bay National Park
St Lucia is also only just over an hour's drive from the stunning Hluhluwe-Imfolozi Game Reserve, where the game viewing is at its best that time of year (as the visibility is better due to sparser vegetation, and the animals congregate around the limited sources of permanent water).
Mentioned in this answer: